Replace Python bootstrapping script with Rust implementation (#2842)

See https://github.com/astral-sh/uv/issues/2617

Note this also includes:
- #2918 
- #2931 (pending)

A first step towards Python toolchain management in Rust.

First, we add a new crate to manage Python download metadata:

- Adds a new `uv-toolchain` crate
- Adds Rust structs for Python version download metadata
- Duplicates the script which downloads Python version metadata
- Adds a script to generate Rust code from the JSON metadata
- Adds a utility to download and extract the Python version

I explored some alternatives like a build script using things like
`serde` and `uneval` to automatically construct the code from our
structs but deemed it to heavy. Unlike Rye, I don't generate the Rust
directly from the web requests and have an intermediate JSON layer to
speed up iteration on the Rust types.

Next, we add add a `uv-dev` command `fetch-python` to download Python
versions per the bootstrapping script.

- Downloads a requested version or reads from `.python-versions`
- Extracts to `UV_BOOTSTRAP_DIR`
- Links executables for path extension

This command is not really intended to be user facing, but it's a good
PoC for the `uv-toolchain` API. Hash checking (via the sha256) isn't
implemented yet, we can do that in a follow-up.

Finally, we remove the `scripts/bootstrap` directory, update CI to use
the new command, and update the CONTRIBUTING docs.

<img width="1023" alt="Screenshot 2024-04-08 at 17 12 15"
src="57bd3cf1-7477-4bb8-a8e9-802a00d772cb">

crates/uv-extract/src/stream.rs

@@ -157,6 +157,7 @@ pub async fn untar_gz<R: tokio::io::AsyncRead + Unpin>(
 ) -> Result<(), Error> {
     let reader = tokio::io::BufReader::new(reader);
     let decompressed_bytes = async_compression::tokio::bufread::GzipDecoder::new(reader);
+
     let mut archive = tokio_tar::ArchiveBuilder::new(decompressed_bytes)
         .set_preserve_mtime(false)
         .build();