Add gradle validation (#748)

Summary:
Copied from here: https://github.com/facebook/fresco/pull/2457

Our version is too old to have checksums available, so we first need to merge
https://github.com/facebook/flipper/pull/749.
Pull Request resolved: https://github.com/facebook/flipper/pull/748

Test Plan: GitHub

Reviewed By: mweststrate

Differential Revision: D19447711

Pulled By: passy

fbshipit-source-id: acd1752fa84c14804f0e1012e56763635127d926

.github/workflows/gradle-wrapper-validation.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