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