ci: unmanaged dependency check

This is is the implementation for the one I got your review in
go/cloud-sdk-java-dependency-governance-design. This check will
avoid accidentally adding a third-party dependency.

b/320677249

.github/workflows/unmanaged_dependency_check.yaml

@@ -0,0 +1,22 @@
+on:
+  pull_request:
+name: Unmanaged dependency check
+jobs:
+  unmanaged_dependency_check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-java@v3
+        with:
+          distribution: temurin
+          java-version: 11
+      - name: Install modules
+        shell: bash
+        run: |
+          # No argument to build.sh installs the modules in local Maven
+          # repository
+          .kokoro/build.sh
+      - name: Unmanaged dependency check
+        uses: googleapis/sdk-platform-java/java-shared-dependencies/unmanaged-dependency-check@unmanaged-dependencies-check-latest
+        with:
+          bom-path: google-cloud-pubsub-bom/pom.xml