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/workspace/src/workspace.rs

@@ -1153,6 +1153,14 @@ impl Workspace {
                 DB.next_id().await.unwrap_or_else(|_| Default::default())
             };
 
+            let toolchains = DB.toolchains(workspace_id).await?;
+            for (toolchain, worktree_id) in toolchains {
+                project_handle
+                    .update(&mut cx, |this, cx| {
+                        this.activate_toolchain(worktree_id, toolchain, cx)
+                    })?
+                    .await;
+            }
             let window = if let Some(window) = requesting_window {
                 cx.update_window(window.into(), |_, cx| {
                     cx.replace_root_view(|cx| {
@@ -5522,6 +5530,14 @@ pub fn open_ssh_project(
             )
         })?;
 
+        let toolchains = DB.toolchains(workspace_id).await?;
+        for (toolchain, worktree_id) in toolchains {
+            project
+                .update(&mut cx, |this, cx| {
+                    this.activate_toolchain(worktree_id, toolchain, cx)
+                })?
+                .await;
+        }
         let mut project_paths_to_open = vec![];
         let mut project_path_errors = vec![];