toolchains: Allow users to provide custom paths to toolchains (#37009)

- **toolchains: Add new state to toolchain selector**
- **Use toolchain term for Add Toolchain button**
- **Hoist out a meta function for toolchain listers**

Closes #27332

Release Notes:

- python: Users can now specify a custom path to their virtual
environment from within the picker.

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/workspace/src/persistence/model.rs

@@ -5,12 +5,14 @@ use crate::{
 };
 use anyhow::Result;
 use async_recursion::async_recursion;
+use collections::IndexSet;
 use db::sqlez::{
     bindable::{Bind, Column, StaticColumnCount},
     statement::Statement,
 };
 use gpui::{AsyncWindowContext, Entity, WeakEntity};
 
+use language::{Toolchain, ToolchainScope};
 use project::{Project, debugger::breakpoint_store::SourceBreakpoint};
 use remote::RemoteConnectionOptions;
 use std::{
@@ -57,6 +59,7 @@ pub(crate) struct SerializedWorkspace {
     pub(crate) docks: DockStructure,
     pub(crate) session_id: Option<String>,
     pub(crate) breakpoints: BTreeMap<Arc<Path>, Vec<SourceBreakpoint>>,
+    pub(crate) user_toolchains: BTreeMap<ToolchainScope, IndexSet<Toolchain>>,
     pub(crate) window_id: Option<u64>,
 }