Duotone: Enable Custom duotones for Global Styles

When #49838 was merged we had to disable the option to edit custom duotone filters on the Global Styles level because it isn't supported. It also isn't supported in theme.json at all, we should add this option, since right now it's only possible on the block level.

Coming back to this one, I'm going to close this one as covered by #81605. If in error, we'll reopen!