Use rustls-tls with manual SSL_CERT_FILE implementation

[charliermarsh]

Summary

This is one solution to resolving #2346: use rustls-tls, but continue to allow SSL_CERT_FILE.

On my machine, it cuts simple commands dramatically.

On main:

❯ echo "requests" | ./target/release/uv pip compile -
Resolved 5 packages in 123ms

On this branch:

❯ echo "requests" | ./target/release/uv pip compile -
Resolved 5 packages in 4ms

I'm sure there are other considerations here but it's an option.

[zanieb]

Hm, that's a bummer. Can we toggle using system certificates with a flag instead and have it be off by default? I'm hesitant to require SSL_CERT_FILE and just using the system trust store is superior in a lot of cases. I'd maybe even suggest it be on by default... but the performance regression is pretty large.

[charliermarsh]

I can try that, yeah.

[charliermarsh]

Closing in favor of #2362.