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/extension_host/src/wasm_host.rs

@@ -1,11 +1,11 @@
 pub mod wit;
 
-use crate::{ExtensionManifest, ExtensionRegistrationHooks};
+use crate::ExtensionManifest;
 use anyhow::{anyhow, bail, Context as _, Result};
 use async_trait::async_trait;
 use extension::{
-    CodeLabel, Command, Completion, KeyValueStoreDelegate, ProjectDelegate, SlashCommand,
-    SlashCommandArgumentCompletion, SlashCommandOutput, Symbol, WorktreeDelegate,
+    CodeLabel, Command, Completion, ExtensionHostProxy, KeyValueStoreDelegate, ProjectDelegate,
+    SlashCommand, SlashCommandArgumentCompletion, SlashCommandOutput, Symbol, WorktreeDelegate,
 };
 use fs::{normalize_path, Fs};
 use futures::future::LocalBoxFuture;
@@ -40,7 +40,7 @@ pub struct WasmHost {
     release_channel: ReleaseChannel,
     http_client: Arc<dyn HttpClient>,
     node_runtime: NodeRuntime,
-    pub registration_hooks: Arc<dyn ExtensionRegistrationHooks>,
+    pub(crate) proxy: Arc<ExtensionHostProxy>,
     fs: Arc<dyn Fs>,
     pub work_dir: PathBuf,
     _main_thread_message_task: Task<()>,
@@ -330,7 +330,7 @@ impl WasmHost {
         fs: Arc<dyn Fs>,
         http_client: Arc<dyn HttpClient>,
         node_runtime: NodeRuntime,
-        registration_hooks: Arc<dyn ExtensionRegistrationHooks>,
+        proxy: Arc<ExtensionHostProxy>,
         work_dir: PathBuf,
         cx: &mut AppContext,
     ) -> Arc<Self> {
@@ -346,7 +346,7 @@ impl WasmHost {
             work_dir,
             http_client,
             node_runtime,
-            registration_hooks,
+            proxy,
             release_channel: ReleaseChannel::global(cx),
             _main_thread_message_task: task,
             main_thread_message_tx: tx,