Automate Maven Central publishing in release workflow

[jeremyprime]

Summary

Automate publishing to Maven Central as part of the CI release workflow. Currently, CI generates artifacts and a GitHub release, but the Maven Central publish is manual.

Should follow the approach used by valkey-glide, GPG-sign artifacts, generate checksums, bundle into correct directory structure, and upload via the Sonatype Central Publisher API. Requires adding repo secrets for Sonatype credentials and GPG signing key.