feat: support config-settings

[henryiii]

Addresses #1227. Don't have much for tests yet, open to ideas.

[joerick]

This looks good to me! Only thing I'm wondering about is, since the values use double-quotes and shlex.split, what happens with a config like this:

[tool.cibuildwheel.config-settings]
example = "a string with \"some quotes\" in it"

I don't know if that works, but... maybe that's an evil/unrealistic input, so if it's awkward to support we could just raise an error when a config-settings value has a double-quote in it (and request the user raise an issue on the tracker).

[henryiii]

Would it be better to use shlex.quote? _inner_fmt could provide the quoted value too, as quoted_v or something.

[joerick]

Yeah I think shlex.quote would work better. I think that shlex.split and shlex.quote should roundtrip perfectly.

[joerick]

Oh nice. I hadn't realised the shlex.quote fix would apply to environment too.