Add GitHub action to verify gradle-wrapper.jar (#3503)

Checks that the gradle wrapper jar is from an official release.
See https://github.com/gradle/wrapper-validation-action for more details.

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

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

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: Apache-2.0
+name: "Validate Gradle Wrapper"
+on: [push, pull_request]
+
+jobs:
+  validation:
+    name: "Gradle Wrapper Validation"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: gradle/wrapper-validation-action@v1