Verify Gradle on each pull request (corona-warn-app#475)

Co-authored-by: Hee Tatt Ooi <hee.tatt.ooi@sap.com>
Co-authored-by: Jakob Möller <jakob.moeller@sap.com>
Co-authored-by: harambasicluka <64483219+harambasicluka@users.noreply.github.com>

.github/workflows/gradle-wrapper-validation.yml

@@ -0,0 +1,17 @@
+name: "Validate Gradle Wrapper"
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  validation:
+    name: "Validation"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: gradle/wrapper-validation-action@v1