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/collab/src/tests/test_server.rs

@@ -17,6 +17,7 @@ use collab_ui::channel_view::ChannelView;
 use collections::{HashMap, HashSet};
 use fs::FakeFs;
 use futures::{channel::oneshot, StreamExt as _};
+use git::GitHostingProviderRegistry;
 use gpui::{BackgroundExecutor, Context, Model, Task, TestAppContext, View, VisualTestContext};
 use language::LanguageRegistry;
 use node_runtime::FakeNodeRuntime;
@@ -257,6 +258,11 @@ impl TestServer {
                 })
             });
 
+        let git_hosting_provider_registry =
+            cx.update(|cx| GitHostingProviderRegistry::default_global(cx));
+        git_hosting_provider_registry
+            .register_hosting_provider(Arc::new(git_hosting_providers::Github));
+
         let fs = FakeFs::new(cx.executor());
         let user_store = cx.new_model(|cx| UserStore::new(client.clone(), cx));
         let workspace_store = cx.new_model(|cx| WorkspaceStore::new(client.clone(), cx));