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/languages/Cargo.toml

@@ -47,6 +47,11 @@ log.workspace = true
 lsp.workspace = true
 node_runtime.workspace = true
 paths.workspace = true
+pet.workspace = true
+pet-core.workspace = true
+pet-conda.workspace = true
+pet-poetry.workspace = true
+pet-reporter.workspace = true
 project.workspace = true
 regex.workspace = true
 rope.workspace = true