Enable environment variable authentication for named indexes

[charliermarsh]

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:

[[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:

export UV_INDEX_INTERNAL_USERNAME=public
export UV_INDEX_INTERNAL_PASSWORD=koala