Enable wrapper validation by default

- Add 'allow-snapshot-wrappers' input parameter
- Default 'validate-wrappers' to 'true'

Fixes #12

.github/workflows/integ-test-wrapper-validation.yml

@@ -29,8 +29,8 @@ jobs:
     - name: Run wrapper-validation-action
       id: setup-gradle
       uses: ./setup-gradle
-      with:
-        validate-wrappers: true
+      env:
+        ALLOWED_GRADLE_WRAPPER_CHECKSUMS: ''
       continue-on-error: true
 
     - name: Check failure