Add a registry for GitHostingProviders (#11470)

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

crates/git/src/git.rs

@@ -1,5 +1,4 @@
 mod hosting_provider;
-mod hosting_providers;
 
 use anyhow::{anyhow, Context, Result};
 use serde::{Deserialize, Serialize};
@@ -11,7 +10,6 @@ pub use git2 as libgit;
 pub use lazy_static::lazy_static;
 
 pub use crate::hosting_provider::*;
-pub use crate::hosting_providers::*;
 
 pub mod blame;
 pub mod commit;