add CI job to validate commit messages

I'm not sure why it seems more frequent lately but more contributions seem to be coming in with patches that don't fit the project's standards. Historically, we've been fairly relaxed about some of this stuff but we should document and codify some of the practices we've been following as well as consider implementing actual GH actions to validate (where reasonable) them.

Existing practices to standardize on:
* Requiring a Signed-off-by trailer
* Requiring topical style commit subject (`cephfs: add foo` or `rados: fix foo` style)

Any thing else to add to the list?

The issue names no files or tests. Start by locating the repository's contribution guidance and GitHub Actions configuration, then determine how signed-off trailers and topical commit subjects should be documented and checked. Done means the practices are documented and a CI job validates the agreed rules.

Written by the indexing model from the issue text.