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="https://github.com/astral-sh/uv/assets/2586601/57bd3cf1-7477-4bb8-a8e9-802a00d772cb">

.github/workflows/ci.yml

@@ -72,33 +72,23 @@ jobs:
     name: "cargo test | ${{ matrix.os }}"
     steps:
       - uses: actions/checkout@v4
-
-      - if: ${{ matrix.os == 'macos' }}
-        name: "Install bootstrap dependencies"
-        run: brew install coreutils
-
-      - uses: actions/setup-python@v5
-        with:
-          python-version: "3.12"
-
-      - name: "Install required Python versions"
-        run: |
-          python -m pip install "zstandard==0.22.0"
-          python scripts/bootstrap/install.py
-
       - name: "Install Rust toolchain"
         run: rustup show
 
-      - name: "Install cargo nextest"
-        uses: taiki-e/install-action@v2
-        with:
-          tool: cargo-nextest
-
       - if: ${{ matrix.os != 'windows' }}
         uses: rui314/setup-mold@v1
 
       - uses: Swatinem/rust-cache@v2
 
+      - name: "Install required Python versions"
+        run: |
+          cargo run -p uv-dev -- fetch-python
+
+      - name: "Install cargo nextest"
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-nextest
+
       - name: "Cargo test"
         run: |
           cargo nextest run --workspace --status-level skip --failure-output immediate-final --no-fail-fast -j 12 --final-status-level slow