**Description:** Expose `cache-primary-key` as an output to allow integration with `actions/cache`and `actions/cache/restore`. **Justification:** Caching rules differ according to the build tool used (mvn/sbt/etc), and I would like to leverage the logic already implemented on `actions/setup-java` to handle the specifics for each build tool. That being said, I have dependent jobs that I would like to guarantee that the same cache is used by forcing the same cache key. I understand that any advanced cache usage should be done manually using `actions/cache`, but just exposing the cache key used by `actions/setup-java` as an output, like `cache-hit`, would make it possible to extend the functionality from `actions/setup-java`. Another possibility would be for `actions/setup-java` not only to set an output with the cache key but also to accept an input cache-key, but I think this goes into the realm of "should be done using actions/cache". **Are you willing to submit a PR?** My understanding is that a small change [here](https://github.com/actions/setup-java/blob/7a445ee88d4e23b52c33fdc7601e40278616c7f8/src/cache.ts#L113) to set an output, and to `action.yml` to declare the output, would suffice. I would be more than happy to submit a PR, but before doing so, I would like to check with the team to see if this aligns with the vision for the future.