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-interpreter/src/interpreter.rs

@@ -16,6 +16,7 @@ use platform_tags::{Tags, TagsError};
 use pypi_types::Scheme;
 use uv_cache::{Cache, CacheBucket, CachedByTimestamp, Freshness, Timestamp};
 use uv_fs::{write_atomic_sync, PythonExt, Simplified};
+use uv_toolchain::PythonVersion;
 
 use crate::Error;
 use crate::Virtualenv;
@@ -314,6 +315,18 @@ impl Interpreter {
             },
         }
     }
+
+    /// Check if the interpreter matches the given Python version.
+    ///
+    /// If a patch version is present, we will require an exact match.
+    /// Otherwise, just the major and minor version numbers need to match.
+    pub fn satisfies(&self, version: &PythonVersion) -> bool {
+        if version.patch().is_some() {
+            version.version() == self.python_version()
+        } else {
+            (version.major(), version.minor()) == self.python_tuple()
+        }
+    }
 }
 
 /// The `EXTERNALLY-MANAGED` file in a Python installation.