Cache managed Python archive downloads
Part of #11834
Split the download-and-extract of manged Python interpreters into a download to the cache and an extract-and-install phase. This allows quickly re-installing Python interpreters when they are cached, and caching Python installation in tests and CI (follow-up PR).
Ideally, we would still have a combined download-and-extract step if the interpreter is available locally, but I couldn't figure out how to tee the stream in reasonable complexity. I nearly succeeded going through a futures `Stream`, but I couldn't figure out how to pass the second writer to the `Stream::then` `FnMut` in a way the borrow checker would accept.
Locally for me, `cargo test -p uv -- python_install` goes from 43s to 7s when setting `UV_PYTHON_CACHE_DIR`.
