RFR: 8263204: Add Gradle Wrapper Validation Action

[John Neffenger]

See the [Gradle Wrapper Validation Action](https://github.com/marketplace/actions/gradle-wrapper-validation) for details on this pull request. I'll test the changes with the following sequence of commits:

1. This commit adds a tampered Gradle Wrapper JAR file, which should go undetected.
2. The next commit will add the Official Gradle Wrapper Validation Action, which should detect the tampered file.
3. The final commit will remove the tampered file and replace it with the original Gradle 4.8 Wrapper.

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Commit messages:
 - 8263204: Add Gradle Wrapper Validation Action

Changes: https://git.openjdk.java.net/jfx/pull/419/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=419&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263204
  Stats: 0 lines in 1 file changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/419.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/419/head:pull/419

PR: https://git.openjdk.java.net/jfx/pull/419