Refactor Git hosting providers (#11457)

This PR refactors the code pertaining to Git hosting providers to make
it more uniform and easy to add support for new providers.

There is now a `GitHostingProvider` trait that contains the
functionality specific to an individual Git hosting provider. Each
provider we support has an implementation of this trait.

Release Notes:

- N/A

Cargo.lock

@@ -4384,6 +4384,7 @@ name = "git"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "clock",
  "collections",
  "git2",