Add border controls to featured image

[jameskoster]

This PR makes it possible to add borders and border-radii to the featured image block:

There is a quirk with this (most basic) implementation. The radius is applied to the figure, and so the img doesn't receive the properties. This feels a bit unexpected when you visit the frontend and see this:

Note how the image corners are overlapping the border.

Despite the fact this is working as intended, we may want to consider adding overflow: hidden to any block that also has a border radius, to ensure that its contents respect that radius. E.g.:

[aaronrobertshaw]

Thanks for assigning this one to me @jameskoster 👍

Rather than attempting to force additional styles such as overflow: hidden onto any block that may adopt border-radius support, I think we might be better served by applying the border styles for this block directly on the inner img element. One downside of forcing overflow: hidden on blocks at the wrapper level is that it can also hide the inline inserter when inner blocks can be added.

I've put together a separate PR illustrating my suggested approach so it can be tested and compared easily alongside this one.

Let me know what you think and if that meets your needs and expectations.

[jameskoster]

Thanks for assigning this one to me @jameskoster 👍

Oops, I meant to request a review and assigned accidentally. Sorry about that.

I've put together a #42847

Nice, I'll take a look.

[aaronrobertshaw]

Closing this as borders were added via #42847.