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/proto/src/proto.rs

@@ -358,7 +358,12 @@ messages!(
     (LanguageServerPromptResponse, Foreground),
     (GitBranches, Background),
     (GitBranchesResponse, Background),
-    (UpdateGitBranch, Background)
+    (UpdateGitBranch, Background),
+    (ListToolchains, Foreground),
+    (ListToolchainsResponse, Foreground),
+    (ActivateToolchain, Foreground),
+    (ActiveToolchain, Foreground),
+    (ActiveToolchainResponse, Foreground)
 );
 
 request_messages!(
@@ -475,7 +480,10 @@ request_messages!(
     (FlushBufferedMessages, Ack),
     (LanguageServerPromptRequest, LanguageServerPromptResponse),
     (GitBranches, GitBranchesResponse),
-    (UpdateGitBranch, Ack)
+    (UpdateGitBranch, Ack),
+    (ListToolchains, ListToolchainsResponse),
+    (ActivateToolchain, Ack),
+    (ActiveToolchain, ActiveToolchainResponse)
 );
 
 entity_messages!(
@@ -555,7 +563,10 @@ entity_messages!(
     GetPermalinkToLine,
     LanguageServerPromptRequest,
     GitBranches,
-    UpdateGitBranch
+    UpdateGitBranch,
+    ListToolchains,
+    ActivateToolchain,
+    ActiveToolchain
 );
 
 entity_messages!(