Support extras in the action

[henryiii]

How would this feature be useful?

Currently, the action doesn't install uv. There's not a way to inject it, either.

Describe the solution you'd like

I think the most general solution would be to support extras. So you could specify extras: uv or extras: tox-to-nox in the action inputs.

Describe alternatives you've considered

I thought about a uv: true setting, but I think the extras method is more general, also supports the other existing extra, and could be used in the future.

I also thought about adding uv by default, but that would limit the action to only binary-supported platforms (which is all the default runners, admittedly, but might not cover some self-hosted runners, perhaps? It's also a tiny extra install for users who aren't using it.

Anything else?

Might be best to make sure this is in #768, as I think we need that for the action. This will very likely need the fix for #791 too.