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-dev/src/main.rs

@@ -21,6 +21,7 @@ use resolve_many::ResolveManyArgs;
 use crate::build::{build, BuildArgs};
 use crate::clear_compile::ClearCompileArgs;
 use crate::compile::CompileArgs;
+use crate::fetch_python::FetchPythonArgs;
 use crate::render_benchmarks::RenderBenchmarksArgs;
 use crate::resolve_cli::ResolveCliArgs;
 use crate::wheel_metadata::WheelMetadataArgs;
@@ -44,6 +45,7 @@ static GLOBAL: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;
 mod build;
 mod clear_compile;
 mod compile;
+mod fetch_python;
 mod render_benchmarks;
 mod resolve_cli;
 mod resolve_many;
@@ -72,6 +74,8 @@ enum Cli {
     Compile(CompileArgs),
     /// Remove all `.pyc` in the tree.
     ClearCompile(ClearCompileArgs),
+    /// Fetch Python versions for testing
+    FetchPython(FetchPythonArgs),
 }
 
 #[instrument] // Anchor span to check for overhead
@@ -92,6 +96,7 @@ async fn run() -> Result<()> {
         Cli::RenderBenchmarks(args) => render_benchmarks::render_benchmarks(&args)?,
         Cli::Compile(args) => compile::compile(args).await?,
         Cli::ClearCompile(args) => clear_compile::clear_compile(&args)?,
+        Cli::FetchPython(args) => fetch_python::fetch_python(args).await?,
     }
     Ok(())
 }