Add overlay color support

[gaambo]

Originally reported here: https://wordpress.org/support/topic/cover-block-width-not-working-properly/#post-18102828

For parity with the cover block, allow setting a overlay color when a background image is set would be great. At the moment the slide block supports setting a background color and background image (+position & size) thanks to the background block support.
The overlay color and opacity is a custom implementation in the cover block (see here). The basic implementation would probably not be too complex. There are components like ColorGradientSettingsDropdown to use for selecting a color or gradient.

But the cover block also does some guessing of a good color based on the background image etc - that would have to be replicated/copied over and I don't really like the idea of that.

So maybe implement that in steps.