add CI job to validate commit messages

[phlogistonjohn]

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?

[phlogistonjohn]

Another thing we probably should do is add an LLM code policy. We've gotten a few LLM assisted contributions lately too. I know ceph/ceph is working on a policy. It's not something we can enforce in CI but we should have a stated policy anyway. My preference is to allow generated contributions but require the submitter fully understand the contribution and state where and what LLM tools were used to generate stuff.

We don't need to jump on this right away but it popped into my head when I was thinking about Signed-off-by, as I think that's even more important now with LLMs being used. We can monitor core ceph's progress on this discussion and borrow what is useful for this project.