Enable environment variable authentication for named indexes (#7741)
## Summary
This PR enables users to provide index credentials via named environment
variables.
For example, given an index named `internal` that requires a username
(`public`) and password
(`koala`), you can define the index (without credentials) in your
`pyproject.toml`:
```toml
[[tool.uv.index]]
name = "internal"
url = "https://pypi-proxy.corp.dev/simple"
```
Then set the `UV_INDEX_INTERNAL_USERNAME` and
`UV_INDEX_INTERNAL_PASSWORD`
environment variables, where `INTERNAL` is the uppercase version of the
index name:
```sh
export UV_INDEX_INTERNAL_USERNAME=public
export UV_INDEX_INTERNAL_PASSWORD=koala
```
