Support selection of tox plugins + add tox-gh

[sirosen]

tox-plugins is a new config for an array of installed tox plugins.
It defaults to tox-uv (previously the only installed plugin) and tox-gh.

Users can therefore override and remove the plugins, but normally should not need to do so.

Installation of the tox plugins is made into a separate step for simplicity.

[sirosen]

I've already tested this on my fork of globus-cli in a branch and confirmed that it works and doesn't break CI.

[kurtmckee]

This begins to diverge from the original at kurtmckee/github-workflows. If we're going this direction, that's fine, but this repo does not currently have all of the infrastructure built out to support doing this correctly; the YAML workflow in this repo is a generated artifact and will lack maintainability if it diverges.

I'd like to close this and simply copy the YAML code from kurtmckee/github-workflows wholesale.

[sirosen]

I'm closing this, but between this PR and globus/globus-cli#1227 , I admit that I'm not totally happy.

I think I would have been more positive about this if our attitude had been that globus/globus-cli#1227 could merge with the expectation that we'd either disable tox-gh or else revert those changes when updating the workflow. And I have a preference for not depending on tox-gh because I think it's dependency bloat -- we aren't using its most complex features.

Here's my opinion:

• I don't think tox-gh is worth bothering to use
• I prefer to have the modest additional output from Add lines to make tox output grouped in GitHub globus-cli#1227 added to toxfile.py in various repos

That opinion doesn't need to be shared, but it feels like I'm not getting a fair chance to advocate for that position based on the current setup.

---

Okay, all that said, I also don't need us to diverge this copy away from the upstream one and make a mess that way.

I'll make a PR upstream to add the feature of "you can choose your tox plugins", which is what I mentioned in #21 as being, IMO, desirable. I'd like to be able to turn off tox-gh if I so choose, without having to give up on the reusable workflow altogether. If we can't agree on making tox-gh optional, I would start to have second thoughts entirely about the use of reusable workflows for our tox invocations.