chore: add github action to generate API index (#654)

* chore: add github action to generate API index

* chore: check diff before attempting commit

* chore: update repo target

.github/workflows/generate_api_index.yaml

@@ -0,0 +1,30 @@
+name: Generate API Index
+on:
+  push:
+    branches: [master]
+  workflow_dispatch:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Setup .NET Core SDK
+        uses: actions/setup-dotnet@v1.8.0
+      - name: Checkout googleapis (this repository)
+        uses: actions/checkout@v2
+      - name: Checkout index generator
+        uses: actions/checkout@v2
+        with:
+          repository: googleapis/googleapis-api-index-generator
+          path: gen
+      - name: Generate API index
+        run: |
+          gen/scripts/generate-schema.sh
+          gen/scripts/generate-index.sh $PWD
+          cp gen/tmp/api-index-v1.json $PWD
+      - name: Commit API index
+        run: |
+          [[ ! $(git diff --exit-code api-index-v1.json) ]] && echo "Nothing to commit." && exit 0
+          git config user.name "Google APIs"
+          git add api-index-v1.json
+          git commit -m "chore: regenerate API index"
+          git push