Add semgrep scanning configuration (#1571)

Runs a nightly semgrep scan.

.github/workflows/semgrep-analysis.yml

@@ -0,0 +1,27 @@
+name: Semgrep
+
+on:
+  schedule:
+    - cron: '0 2 * * *'
+
+jobs:
+  semgrep:
+    name: Scan
+    runs-on: ubuntu-latest
+    steps:
+      # Checkout project source
+      - uses: actions/checkout@v2
+
+      # Scan code using project's configuration on https://semgrep.dev/manage
+      - uses: returntocorp/semgrep-action@v1
+        with:
+          publishToken: ${{ secrets.SEMGREP_APP_TOKEN }}
+          publishDeployment: ${{ secrets.SEMGREP_DEPLOYMENT_ID }}
+          generateSarif: "1"
+
+      # Upload SARIF file generated in previous step
+      - name: Upload SARIF file
+        uses: github/codeql-action/upload-sarif@v1
+        with:
+          sarif_file: semgrep.sarif
+        if: always()