Add release automation with GitHub Actions

Releasing this SDK requires dealing with Maven and GPG, both of which can be finicky and difficult. We should have a manual workflow that we can run from the repo's GitHub Actions that will kick off the release for us. I assume we'll still have to do the final step on oss.sonatype.com manually, but that could be a good thing because we'll be able to verify that it worked correctly.

Steps for release are outlined in RELEASING.md

Start by reading RELEASING.md to identify the Maven and GPG release steps and the point where oss.sonatype.com remains manual. Then review the repository's GitHub Actions configuration and define a manually triggered workflow that performs the documented release steps; done means the workflow can be run from GitHub Actions and leaves only the intended final Sonatype step.

Written by the indexing model from the issue text.