Spacer: Add background color support

[hanneslsm]

There is a PR to this feature already, but it was closed without a proper discussion.
I strongly disagree with closing the PR. Therefore, this is a separate issue for the background color support for the spacer block. This feature is very much needed.

What problem does this address?

I often need an area with a background color but no content.
It's important that the size of this area can be defined with spacing sizes, because they use clamp.
The group block is not a viable solution, as outlined in my comment: #66433 (comment) The spacer is much better suited here, therefore, let's just add the design tools.

Also, a strong argument for adding the design tools is consistency in the editor, see the corresponding tracking issue: #43245

Other arguments for adding design tools to the spacer block were outlined in #43247 (comment) and #66443

What is your proposed solution?

Add the background-color design tool to the Spacer block, as in #66433