Overhaul extension registration (#21083)

This PR overhauls extension registration in order to make it more
modular.

The `extension` crate now contains an `ExtensionHostProxy` that can be
used to register various proxies that the extension host can use to
interact with the rest of the system.

There are now a number of different proxy traits representing the
various pieces of functionality that can be provided by an extension.
The respective crates that provide this functionality can implement
their corresponding proxy trait in order to register a proxy that the
extension host will use to register the bits of functionality provided
by the extension.

Release Notes:

- N/A

crates/collab/src/tests/remote_editing_collaboration_tests.rs

@@ -1,6 +1,7 @@
 use crate::tests::TestServer;
 use call::ActiveCall;
 use collections::HashSet;
+use extension::ExtensionHostProxy;
 use fs::{FakeFs, Fs as _};
 use futures::StreamExt as _;
 use gpui::{BackgroundExecutor, Context as _, SemanticVersion, TestAppContext, UpdateGlobal as _};
@@ -81,6 +82,7 @@ async fn test_sharing_an_ssh_remote_project(
                 http_client: remote_http_client,
                 node_runtime: node,
                 languages,
+                extension_host_proxy: Arc::new(ExtensionHostProxy::new()),
             },
             cx,
         )
@@ -243,6 +245,7 @@ async fn test_ssh_collaboration_git_branches(
                 http_client: remote_http_client,
                 node_runtime: node,
                 languages,
+                extension_host_proxy: Arc::new(ExtensionHostProxy::new()),
             },
             cx,
         )
@@ -400,6 +403,7 @@ async fn test_ssh_collaboration_formatting_with_prettier(
                 http_client: remote_http_client,
                 node_runtime: NodeRuntime::unavailable(),
                 languages,
+                extension_host_proxy: Arc::new(ExtensionHostProxy::new()),
             },
             cx,
         )