Global Styles: add Caption element UI controls

[MaggieCabrera]

#41140 Introduced the Caption element to be controlled via theme.json, we should introduce a UI in Global Styles so it can be editable by users too, the same way we did for the heading and button elements.

[github-actions]

Hi,
This issue has gone 30 days without any activity. This means it is time for a check-in to make sure it is still relevant. If you are still experiencing this issue with the latest versions, you can help the project by responding to confirm the problem and by providing any updated reproduction steps.
Thanks for helping out.

[github-actions]

This issue has gone 30 days without any activity.

[richtabor]

Just chiming in that I ran into this limitation while experimenting with theming in the editor.