Support index declarations in uv publish

[konstin]

We want to support uv publish --index example with:

[[index]]
name = "example"
url = "https://pypi.org/simple"
publish-url = "https://upload.pypi.org/legacy"

This avoids setting the options on the CLI, moving them into configuration stored with the project instead.

It requires changing uv publish so that if --index <name> is given, we discover (and require) pyproject.toml to be present.

See also #8531 (comment)

Todo list:

• Add publish-url option to [[index]]
• Add --index option, conflicting with --publish-url and --check-url.
• When using --index, perform workspace discovery, find a matching index and use that
• Update the docs