Provide an way to provide Git authentication for tool.uv.sources

[zanieb]

Right now, the credentials either need to be pre-populated in the git credential helper, or hard-coded into the pyproject.toml. We should provide a better way to do this.

Maybe #5734 is the solution.

[rosssgill]

Hi @zanieb I'm currently experiencing this issue while authenticating with personal access tokens and I would rather not expose this token in source code. Is it possible to respect the UV_GITHUB_TOKEN environment variable if trying to access a private repository when credentials are not provided but this variable is set?