Change 100's Result DNS as pointer to remove empty DNS field

This commit changes DNS as pointer to remove empty DNS field in
Result. This will simplify Result format. This also changes
SPEC to mention all keys are optional, other than 'cniVerison'.

Signed-off-by: Tomofumi Hayashi <tohayash@redhat.com>

SPEC.md

@@ -523,6 +523,7 @@ For certain operations, plugins must output result information. The output shoul
 ### ADD Success
 
 Plugins must output a JSON object with the following keys upon a successful `ADD` operation:
+All keys are optional other than `cniVersion`.
 
 - `cniVersion`: The same version supplied on input - the string "1.1.0"
 - `interfaces`: An array of all interfaces created by the attachment, including any host-level interfaces:

pkg/types/100/types.go

@@ -92,7 +92,7 @@ type Result struct {
 	Interfaces []*Interface   `json:"interfaces,omitempty"`
 	IPs        []*IPConfig    `json:"ips,omitempty"`
 	Routes     []*types.Route `json:"routes,omitempty"`
-	DNS        types.DNS      `json:"dns,omitempty"`
+	DNS        *types.DNS     `json:"dns,omitempty"`
 }
 
 // convertFrom100 does nothing except set the version; the types are the same
@@ -145,7 +145,7 @@ func convertFrom04x(from types.Result, toVersion string) (types.Result, error) {
 	fromResult := from.(*types040.Result)
 	toResult := &Result{
 		CNIVersion: toVersion,
-		DNS:        *fromResult.DNS.Copy(),
+		DNS:        fromResult.DNS.Copy(),
 		Routes:     []*types.Route{},
 	}
 	for _, fromIntf := range fromResult.Interfaces {

pkg/types/types.go

@@ -64,7 +64,7 @@ type NetConf struct {
 	Type         string          `json:"type,omitempty"`
 	Capabilities map[string]bool `json:"capabilities,omitempty"`
 	IPAM         IPAM            `json:"ipam,omitempty"`
-	DNS          DNS             `json:"dns,omitempty"`
+	DNS          *DNS            `json:"dns,omitempty"`
 
 	RawPrevResult map[string]interface{} `json:"prevResult,omitempty"`
 	PrevResult    Result                 `json:"-"`