Chore: Add support for sending build to firebase app distribution (#647)

Fixes ooni/probe#2643

## Proposed Changes

  - Adds Credentials to the repo
- Add a new workflow file for Firebase App Distribution which publishes
debug `apk`.

.github/workflows/firebase-app-distribution.yml

@@ -0,0 +1,25 @@
+name: Build & upload to Firebase App Distribution
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          java-version: '17'
+          distribution: 'temurin'
+      - name: build release
+        run: ./gradlew clean assembleDevFullDebug
+      - name: upload artifact to Firebase App Distribution
+        uses: wzieba/Firebase-Distribution-Github-Action@v1.7.0
+        with:
+          appId: ${{secrets.FIREBASE_APP_ID}}
+          serviceCredentialsFileContent: ${{ secrets.CREDENTIAL_FILE_CONTENT }}
+          groups: testers
+          file: app/build/outputs/apk/devFull/debug/app-dev-full-debug.apk