Add spacing controls to the featured image block

[kjellr]

This PR adds spacing controls (margin + padding) to the featured image block, so that block themes have more control over placement.

Screenshot:

Testing:

1. Add a featured image block.
2. Check to see if margin and padding controls are active.

(Your theme will need to opt in to showing both Margin and Padding controls. Twenty Twenty-Two is an example of a theme that does this).