Add GitHub Actions workflow to validate the Gradle wrapper

[TheMrMilchmann]

This PR adds a GitHub Actions workflow to validate the (binary) gradle-wrapper.jar. This is achieved by using the official Gradle Wrapper Validation Action from the Gradle team.

The motivation behind this change is to provide better protection against potentially malicious contributions. As an added bonus, reviewing contributions such as #693 gets significantly easier. However, the action only verifies the gradle-wrapper.jar and not the gradlew scripts that are frequently updated together with the wrapper.

Also, note that this should eventually be merged into a general CI workflow if one is added to this repository.

An example run can be found here.