perf: Add full program benchmark for kustomize build

This change introduces a benchmarking test that constructs a
complete kustomization tree using various features of Kustomize.

This update aims to address several objectives:
* Demonstrating current performance challenges in Kustomize in a reproducible manner.
* Evaluating the effects of performance enhancements.
* Guarding against potential performance setbacks and inadvertent quadratic behavior in the future.
* Considering the possibility of incorporating profile-guided optimization (PGO) in future iterations.

Usage:

    go test -run=x -bench=BenchmarkBuild ./kustomize/commands/build

    pkg: sigs.k8s.io/kustomize/kustomize/v5/commands/build
    BenchmarkBuild-8               1        48385043792 ns/op
    PASS
    ok      sigs.k8s.io/kustomize/kustomize/v5/commands/build       48.701s

*Currently*, this benchmark requires 48 seconds to run on my machine.

Updates kubernetes-sigs#5084

Notes on PGO:

Real-life profiles would be better, but creating one based on a benchmark should not hurt:

https://go.dev/doc/pgo#collecting-profiles

> Will PGO with an unrepresentative profile make my program slower than no PGO?
> It should not. While a profile that is not representative of production behavior will result in optimizations in cold parts of the application, it should not make hot parts of the application slower. If you encounter a program where PGO results in worse performance than disabling PGO, please file an issue at https://go.dev/issue/new.

Collecting a profile:

    go test -cpuprofile cpu1.pprof -run=^$ -bench ^BenchmarkBuild$ sigs.k8s.io/kustomize/kustomize/v5/commands/build

    go build -pgo=./cpu1.pprof  -o kust-pgo ./kustomize
    go build -o kust-nopgo ./kustomize

Compare PGO and non-PGO-builds:

    ./kust-pgo build -o /dev/null testdata/  21.88s user 2.00s system 176% cpu 13.505 total
    ./kust-nopgo build -o /dev/null testdata/  22.76s user 1.98s system 174% cpu 14.170 total

kustomize/commands/build/build_benchmark_test.go

@@ -0,0 +1,196 @@
+// Copyright 2023 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package build_test
+
+import (
+	"bytes"
+	"fmt"
+	"path/filepath"
+	"testing"
+
+	. "sigs.k8s.io/kustomize/kustomize/v5/commands/build"
+	"sigs.k8s.io/kustomize/kyaml/filesys"
+)
+
+// GenConfig configures the generation of a kustomization tree for benchmarking purposes
+type GenConfig struct {
+	// The number of plain file resources to generate
+	fileResources int
+
+	// The number of subdirectories to generate
+	resources int
+
+	// The number of patches to generate
+	patches int
+
+	// Whether to generate a namespace field
+	namespaced bool
+
+	// The name prefix to use (if any)
+	namePrefix string
+
+	// The name suffix to use (if any)
+	nameSuffix string
+
+	// Common labels to use (if any)
+	commonLabels map[string]string
+
+	// Common annotations to use (if any)
+	commonAnnotations map[string]string
+}
+
+func makeKustomization(configs []GenConfig, fSys filesys.FileSystem, path, id string, depth int) error {
+	cfg := configs[depth]
+	if err := fSys.MkdirAll(path); err != nil {
+		return fmt.Errorf("failed to make directory %v: %w", path, err)
+	}
+
+	var buf bytes.Buffer
+	if cfg.namespaced {
+		fmt.Fprintf(&buf, "namespace: %s\n", id)
+	}
+
+	if cfg.namePrefix != "" {
+		fmt.Fprintf(&buf, "namePrefix: %s\n", cfg.namePrefix)
+	}
+
+	if cfg.nameSuffix != "" {
+		fmt.Fprintf(&buf, "nameSuffix: %s\n", cfg.nameSuffix)
+	}
+
+	if len(cfg.commonLabels) > 0 {
+		fmt.Fprintf(&buf, "commonLabels:\n")
+		for k, v := range cfg.commonLabels {
+			fmt.Fprintf(&buf, "  %s: %s\n", k, v)
+		}
+	}
+
+	if len(cfg.commonAnnotations) > 0 {
+		fmt.Fprintf(&buf, "commonAnnotations:\n")
+		for k, v := range cfg.commonAnnotations {
+			fmt.Fprintf(&buf, "  %s: %s\n", k, v)
+		}
+	}
+
+	if cfg.fileResources > 0 || cfg.resources > 0 {
+		fmt.Fprintf(&buf, "resources:\n")
+		for res := 0; res < cfg.fileResources; res++ {
+			fn := fmt.Sprintf("res%d.yaml", res)
+			fmt.Fprintf(&buf, " - %v\n", fn)
+
+			cm := fmt.Sprintf(`kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: %s-%d
+  labels:
+    foo: bar
+  annotations:
+    baz: blatti
+data:
+  k: v
+`, id, res)
+			if err := fSys.WriteFile(filepath.Join(path, fn), []byte(cm)); err != nil {
+				return fmt.Errorf("failed to write file resource: %w", err)
+			}
+		}
+
+		for res := 0; res < cfg.resources; res++ {
+			fn := fmt.Sprintf("res%d", res)
+			fmt.Fprintf(&buf, " - %v\n", fn)
+			if err := makeKustomization(configs, fSys, path+"/"+fn, fmt.Sprintf("%s-%d", id, res), depth+1); err != nil {
+				return fmt.Errorf("failed to make kustomization: %w", err)
+			}
+		}
+	}
+
+	for res := 0; res < cfg.patches; res++ {
+		if res == 0 {
+			fmt.Fprintf(&buf, "patches:\n")
+		}
+
+		// alternate between json and yaml patches to test both kinds
+		if res%2 == 0 {
+			fn := fmt.Sprintf("patch%d.yaml", res)
+			fmt.Fprintf(&buf, " - path: %v\n", fn)
+			cmPatch := fmt.Sprintf(`kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: %s-%d
+data:
+  k: v2
+`, id, res)
+			if err := fSys.WriteFile(filepath.Join(path, fn), []byte(cmPatch)); err != nil {
+				return fmt.Errorf("failed to write patch: %w", err)
+			}
+		} else {
+			fn := fmt.Sprintf("patch%d.json", res)
+			fmt.Fprintf(&buf, ` - path: %v
+   target:
+    version: v1
+    kind: ConfigMap
+    name: %s-%d
+`, fn, id, res-1)
+			patch := `[{"op": "add", "path": "/data/k2", "value": "3"} ]`
+			if err := fSys.WriteFile(filepath.Join(path, fn), []byte(patch)); err != nil {
+				return fmt.Errorf("failed to write patch: %w", err)
+			}
+		}
+	}
+
+	if err := fSys.WriteFile(filepath.Join(path, "kustomization.yaml"), buf.Bytes()); err != nil {
+		return fmt.Errorf("failed to write kustomization.yaml: %w", err)
+	}
+	return nil
+}
+
+func BenchmarkBuild(b *testing.B) {
+	// This benchmark generates a kustomization tree with the following structure:
+	genConfig := []GenConfig{
+		{
+			resources: 2, // two nested resources
+
+			// these operations should be very fast, so lets perform them on a *lot* of resources
+			namePrefix: "foo-",
+			nameSuffix: "-bar",
+			commonLabels: map[string]string{
+				"foo": "bar",
+			},
+			commonAnnotations: map[string]string{
+				"baz": "blatti",
+			},
+		},
+		{
+			// test some more nesting (this could be `apps/` or `components/` directory with 50 apps or components)
+			resources: 50,
+		},
+		{
+			// this should be almost the same as using 150 above and skipping it, but it is currently not, so lets have some more nesting
+			resources: 3,
+		},
+		{
+			// here we have an actual component/app with lots or resources. Typically here we set namespace and have some patches
+			resources:     1,
+			namespaced:    true,
+			fileResources: 30,
+			patches:       10,
+		},
+		{
+			// we can also have a base/ or shared resources included into the namespace
+			fileResources: 2,
+		},
+	}
+
+	fSys := filesys.MakeFsInMemory()
+	if err := makeKustomization(genConfig, fSys, "testdata", "res", 0); err != nil {
+		b.Fatal(err)
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		buffy := new(bytes.Buffer)
+		cmd := NewCmdBuild(fSys, MakeHelp("foo", "bar"), buffy)
+		if err := cmd.RunE(cmd, []string{"./testdata"}); err != nil {
+			b.Fatal(err)
+		}
+	}
+}