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/remote_server/Cargo.toml

@@ -29,6 +29,7 @@ chrono.workspace = true
 clap.workspace = true
 client.workspace = true
 env_logger.workspace = true
+extension.workspace = true
 extension_host.workspace = true
 fs.workspace = true
 futures.workspace = true
@@ -37,6 +38,7 @@ git_hosting_providers.workspace = true
 gpui.workspace = true
 http_client.workspace = true
 language.workspace = true
+language_extension.workspace = true
 languages.workspace = true
 log.workspace = true
 lsp.workspace = true