[core] Add GitHub Action for iOS and Android (#71)

This commit adds 2 new jobs to the existing "Continuous Delivery" GitHub
Action to test the build of the iOS and Android app.

For this we also had to adjust the "build.gradle" file for the Android
app, so that the test build in the GitHub Action is signed with dummy
credentials, since we do not provide the keystore properties.

.github/workflows/continuous-delivery.yaml

@@ -364,3 +364,67 @@ jobs:
         with:
           upload_url: ${{ github.event.release.upload_url }}
           asset_path: app/build/feeddeck-windows-x86_64.zip
+
+  # The "iOS" job builds the Flutter iOS app on every pull request. This is only used to test that the build of the iOS
+  # app works. The artifact of the build isn't uploaded / used.
+  ios:
+    name: iOS
+    runs-on: macos-latest
+    if: github.event_name == 'pull_request' || (github.event_name == 'release' && github.event.action == 'published')
+    defaults:
+      run:
+        working-directory: "app"
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Flutter
+        uses: subosito/flutter-action@v2
+        with:
+          flutter-version: '3.16.0'
+          channel: 'stable'
+          cache: true
+          cache-key: 'flutter-:os:-:channel:-:version:-:arch:-:hash:'
+          cache-path: '${{ runner.tool_cache }}/flutter/:channel:-:version:-:arch:'
+
+      - name: Install Dependencies
+        run: |
+          flutter pub get
+
+      - name: Build
+        run: |
+          flutter config --enable-ios
+          flutter build ipa --no-codesign --release --dart-define SUPABASE_URL=${{ secrets.SUPABASE_PROD_URL }} --dart-define SUPABASE_ANON_KEY=${{ secrets.SUPABASE_PROD_ANON_KEY }} --dart-define SUPABASE_SITE_URL=${{ secrets.SUPABASE_PROD_SITE_URL }} --dart-define GOOGLE_CLIENT_ID=${{ secrets.SUPABASE_PROD_GOOGLE_CLIENT_ID }}
+
+  # The "Android" job builds the Flutter Android app on every pull request. This is only used to test that the build of
+  # the Android app works. The artifact of the build isn't uploaded / used.
+  android:
+    name: Android
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request' || (github.event_name == 'release' && github.event.action == 'published')
+    defaults:
+      run:
+        working-directory: "app"
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Flutter
+        uses: subosito/flutter-action@v2
+        with:
+          flutter-version: '3.16.0'
+          channel: 'stable'
+          cache: true
+          cache-key: 'flutter-:os:-:channel:-:version:-:arch:-:hash:'
+          cache-path: '${{ runner.tool_cache }}/flutter/:channel:-:version:-:arch:'
+
+      - name: Install Dependencies
+        run: |
+          flutter pub get
+
+      - name: Build
+        run: |
+          flutter config --enable-android
+          flutter build appbundle --release --dart-define SUPABASE_URL=${{ secrets.SUPABASE_PROD_URL }} --dart-define SUPABASE_ANON_KEY=${{ secrets.SUPABASE_PROD_ANON_KEY }} --dart-define SUPABASE_SITE_URL=${{ secrets.SUPABASE_PROD_SITE_URL }} --dart-define GOOGLE_CLIENT_ID=${{ secrets.SUPABASE_PROD_GOOGLE_CLIENT_ID }}