ci: release to maven on merge

[calumcalder]

Creates a new github action pipeline that runs on merge to master. The pipeline will:

1. Create a new commit with an updated CHANGELOG.md
2. Tag that commit and push it to master
3. Make a clean checkout of that commit
4. Build, sign, and release all java artefacts to maven

Depends on #1422 to set up the secrets in github

[jzacsh]

looks good (obviously pending the secrets as you said). left you a question on the YAML startswith.

[jzacsh]

I see this is still in draft, but LGTM already. (despite our one basically-resolved comment thread on docs)

[calumcalder]

Rebased