campaigns: check PAT/SSH key configuration on request after credential configuration

Created by: LawnGnome

This came up when we were brainstorming the SSH key provisioning UI flow today: it would be nice to be able to provide a button that checks the current PAT/SSH/whatever configuration against the code host and indicates whether it's valid.

We probably can't do this in the background lest we fall foul of automated banning tools on the code host, but we should be able to provide a button that tries to do a dry run push to the code host and reports back if there's an authentication error.

This definitely isn't on the critical path for anything right now, but it's a really nice user affordance.