Add `--show-urls` and `--only-downloads` to `uv python list`

[zanieb]

These are useful for creating a mirror of the Python downloads for a given uv version, e.g.:

❯ cargo run -q -- python list --show-urls --only-downloads
cpython-3.13.0-macos-aarch64-none     https://github.com/indygreg/python-build-standalone/releases/download/20241008/cpython-3.13.0%2B20241008-aarch64-apple-darwin-install_only_stripped.tar.gz
cpython-3.12.7-macos-aarch64-none     https://github.com/indygreg/python-build-standalone/releases/download/20241008/cpython-3.12.7%2B20241008-aarch64-apple-darwin-install_only_stripped.tar.gz
cpython-3.11.10-macos-aarch64-none    https://github.com/indygreg/python-build-standalone/releases/download/20241008/cpython-3.11.10%2B20241008-aarch64-apple-darwin-install_only_stripped.tar.gz
cpython-3.10.15-macos-aarch64-none    https://github.com/indygreg/python-build-standalone/releases/download/20241008/cpython-3.10.15%2B20241008-aarch64-apple-darwin-install_only_stripped.tar.gz
cpython-3.9.20-macos-aarch64-none     https://github.com/indygreg/python-build-standalone/releases/download/20241008/cpython-3.9.20%2B20241008-aarch64-apple-darwin-install_only_stripped.tar.gz
cpython-3.8.20-macos-aarch64-none     https://github.com/indygreg/python-build-standalone/releases/download/20241002/cpython-3.8.20%2B20241002-aarch64-apple-darwin-install_only_stripped.tar.gz
pypy-3.10.14-macos-aarch64-none       https://downloads.python.org/pypy/pypy3.10-v7.3.17-macos_arm64.tar.bz2
pypy-3.9.19-macos-aarch64-none        https://downloads.python.org/pypy/pypy3.9-v7.3.16-macos_arm64.tar.bz2
pypy-3.8.16-macos-aarch64-none        https://downloads.python.org/pypy/pypy3.8-v7.3.11-macos_arm64.tar.bz2

[zanieb]

I might add some tests for this, but they'll be kind of annoying to update.