Add color transparency support

[strarsis]

Is your feature request related to a problem? Please describe.
Sometimes a color should not be completely opaque but also be semitransparent.
E.g. a slightly transparent group block.

Describe the solution you'd like
Add a transparency option to the color selection.

Describe alternatives you've considered
Add and use a block style, however, this would require to add a block style per block type and combining block styles isn't possible yet, so manual class editing is required, too.

[kjellr]

This issue is very close to #18095, except it's only about the Group block. It would make sense to roll out an opacity control more globally, so I'll close this issue and we can continue tracking in #18095.

Thank you!