add Gradle wrapper validation workflow

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

@@ -0,0 +1,10 @@
+name: "Validate Gradle Wrapper"
+on: [push, pull_request]
+
+jobs:
+  validation:
+    name: "Validation"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: gradle/wrapper-validation-action@v1