[actions] Support Semgrep by Github Actions (sonic-net#116)

**Why I did it**
[Semgrep](https://github.com/returntocorp/semgrep) is a static analysis tool to find security vulnerabilities.
When opening a PR or commtting to PR, Semgrep performs a diff-aware scanning, which scans changed files in PRs.
When merging PR, Semgrep performs a full scan on master branch and report all findings.

Ref: - [Supported Language](https://semgrep.dev/docs/supported-languages/#language-maturity) - [Semgrep Rules](https://registry.semgrep.dev/rule)

**How I did it**
Integrate Semgrep into this repository by committing a job configuration file

.github/workflows/semgrep.yml

@@ -0,0 +1,22 @@
+name: Semgrep
+
+on:
+  pull_request: {}
+  push:
+    branches:
+      - master
+      - '201[7-9][0-1][0-9]'
+      - '202[0-9][0-1][0-9]'
+
+jobs:
+  semgrep:
+    if: github.repository_owner == 'sonic-net'
+    name: Semgrep
+    runs-on: ubuntu-latest
+    container:
+      image: returntocorp/semgrep
+    steps:
+      - uses: actions/checkout@v3
+      - run: semgrep ci
+        env:
+           SEMGREP_RULES: p/default