Initial version of the build release artifacts workflow. (#4287)

* Initial version of the build release artifacts workflow.

This version of the workflow hardcodes the list of sdks to release to
include Firestore only. Also, generated javadoc is not yet equivalent
to what's generated by the regular release process.

Author: rlazo

.github/workflows/build-release-artifacts.yml

@@ -0,0 +1,25 @@
+name: Build Release Artifacts
+
+on:
+  workflow_dispatch:
+  pull_request:
+    branches:
+      - 'releases/**'
+
+jobs:
+  build-artifacts:
+    runs-on: ubuntu-latest
+    env:
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Perform gradle build
+        run: |
+          ./gradlew  firebasePublish -PprojectsToPublish=firebase-firestore -PpublishMode=RELEASE -PincludeFireEscapeArtifacts=true
+      - name: Upload generated artifacts
+        uses: actions/upload-artifact@v2
+        with:
+          name: release_artifacts.zip
+          path: build/*.zip
+          retention-days: 5