This PR adds a registry for `GitHostingProvider`s.
The intent here is to help decouple these provider-specific concerns
from the lower-level `git` crate.
Similar to languages, the Git hosting providers live in the new
`git_hosting_providers` crate.
This work also lays the foundation for if we wanted to allow defining a
`GitHostingProvider` from within an extension. This could be useful if
we wanted to extend the support to work with self-hosted Git providers
(like GitHub Enterprise).
I also took the opportunity to move some of the provider-specific code
out of the `util` crate, since it had leaked into there.
Release Notes:
- N/A
