Add "Set Featured Image" button

[kean]

What problem does this address?

The full version of the editor has the "Set Featured Image" button in the "More" menu for image blocks. The mobile version also needs one.

Technical Notes

The primary way to manage featured images is via the native "Post Settings" screen, as featured images are part of the post metadata and not the post content. Gutenberg Mobile solves it using the following callback:

func gutenbergDidRequestToSetFeaturedImage(for mediaID: Int32)

In the current implementation, the updates are two-directional: the app also communicates the changes to the featured image back to the editor using FeaturedImageDelegate (iOS). It only exists to support the "Featured" badge and the "Remove Featured Image" buttons in the editor. I suggest removing this part because it's error-prone and requires too much effort for too little value. The full web editor also doesn't support it as they seemingly made the same calculation.