Image Captions: expand Styles options allowing changing background and text shadow

[jartes]

What problem does this address?

The image caption background changed in #63471, and some users are willing to customize it because it doesn't fit their images well. Currently, this is only possible using custom CSS, and there's no option to do that with Styles.

What is your proposed solution?

Expand the Styles options for Captions and allow customizing the background added to image captions.

PR #49141 added color and typography controls for Captions, so we can use something similar but for the background / text shadow