Add Gradle Wrapper validation workflow

[pauliesnug]

Verifies that PRs to the gradle/wrapper directory are legitimate.

(By validating the Gradle Wrapper, we can prevent attacks that maliciously modify the gradle wrapper files, see https://blog.gradle.org/wrapper-attack-report)

Description

Related Issue(s)

Checklist

• [x] I made a clear description of what was changed
• [x] I stated why these changes were necessary
• [x] I updated documentation or said what needs to be updated
• [x] I made sure these changes are backwards compatible
• [x] This pull request is for one feature/bug fix