Add S3 request signing (#15925)
## Summary
This PR enables users to mark a URL as an S3 endpoint, at which point uv
will sign requests to that URL by detecting credentials from the
standard AWS environment variables, configuration files, etc.
Signing is handled by the
[reqsign](https://docs.rs/reqsign/latest/reqsign/) crate, which we can
also use in the future to sign requests for other providers.
