feat: mTLS support (#4171)
## Summary
Closes https://github.com/astral-sh/uv/issues/3626
This adds mTLS support to uv via the standard env var `SSL_CLIENT_CERT`.
## Test Plan
Tested locally using a [nginx proxy to
pypi](https://github.com/hauntsaninja/nginx_pypi_cache) using my own
self-signed ca + certs + client certs generated via
[mkcert](https://github.com/FiloSottile/mkcert). Used this proxy with
both uv and pip to make sure we have feature partity in mTLS
functionality.
