Apply from-URL credentials in authentication middleware (#2449)
## Summary
Right now, the middleware doesn't apply credentials that were
_originally_ sourced from a URL. This requires that we call
`with_url_encoded_auth` whenever we create a request to ensure that any
credentials that were passed in as part of an index URL (for example)
are respected.
This PR modifies `uv-auth` to instead apply those credentials in the
middleware itself. This seems preferable to me. As far as I can tell, we
can _only_ add in-URL credentials to the store ourselves (since in-URL
credentials are converted to headers by the time they reach the
middleware). And if we ever _didn't_ apply those credentials to new
URLs, it'd be a bug in the logic that precedes the middleware (i.e., us
forgetting to call `with_url_encoded_auth`).
## Test Plan
`cargo run pip install` with an authenticated index.
