Add language toolchains (#19576)

This PR adds support for selecting toolchains for a given language (e.g.
Rust toolchains or Python virtual environments) with support for SSH
projects provided out of the box. For Python we piggy-back off of
[PET](https://github.com/microsoft/python-environment-tools), a library
maintained by Microsoft.
Closes #16421
Closes #7646

Release Notes:

- Added toolchain selector to the status bar (with initial support for
Python virtual environments)

crates/remote_server/src/headless_project.rs

@@ -10,7 +10,7 @@ use project::{
     search::SearchQuery,
     task_store::TaskStore,
     worktree_store::WorktreeStore,
-    LspStore, LspStoreEvent, PrettierStore, ProjectPath, WorktreeId,
+    LspStore, LspStoreEvent, PrettierStore, ProjectPath, ToolchainStore, WorktreeId,
 };
 use remote::ssh_session::ChannelClient;
 use rpc::{
@@ -108,11 +108,14 @@ impl HeadlessProject {
             observer.shared(SSH_PROJECT_ID, session.clone().into(), cx);
             observer
         });
+        let toolchain_store =
+            cx.new_model(|cx| ToolchainStore::local(languages.clone(), worktree_store.clone(), cx));
         let lsp_store = cx.new_model(|cx| {
             let mut lsp_store = LspStore::new_local(
                 buffer_store.clone(),
                 worktree_store.clone(),
                 prettier_store.clone(),
+                toolchain_store.clone(),
                 environment,
                 languages.clone(),
                 http_client,
@@ -143,6 +146,7 @@ impl HeadlessProject {
         session.subscribe_to_entity(SSH_PROJECT_ID, &cx.handle());
         session.subscribe_to_entity(SSH_PROJECT_ID, &lsp_store);
         session.subscribe_to_entity(SSH_PROJECT_ID, &task_store);
+        session.subscribe_to_entity(SSH_PROJECT_ID, &toolchain_store);
         session.subscribe_to_entity(SSH_PROJECT_ID, &settings_observer);
 
         client.add_request_handler(cx.weak_model(), Self::handle_list_remote_directory);
@@ -166,6 +170,7 @@ impl HeadlessProject {
         SettingsObserver::init(&client);
         LspStore::init(&client);
         TaskStore::init(Some(&client));
+        ToolchainStore::init(&client);
 
         HeadlessProject {
             session: client,