Add GitHub Action to build Java deploy jars

[jin]

Currently, we build the deploy jars under //private/tools/java/... by hand. This PR adds a new GitHub action to build and upload those JARs if the PR contains a change that's transitively depended upon by any of the JARs.

The implementation still requires us to download the zip and upload it in a new PR, but at least it's built from CI, not a local dev machine. Small improvement, that.

A possible next step is to change the upload artifact job into a create PR job, so whenever code changes for the java tools, we get an automated PR that updates the deploy jars in the repo.

Example run: https://github.com/jin/rules_jvm_external/actions/runs/6642435323