Image Compare: Add 'insert from URL' option

#### Is your feature request related to a problem? Please describe.
When adding a regular image, you can choose 'insert from URL' to add any image URL there.

But when adding an Image Compare block, you can only add from the media library or Google/Pexels

#### Describe the solution you'd like
Add an 'insert from URL' option when adding images to the Image Compare block.

#### Additional context
Requested here: https://wordpress.org/support/topic/image-compare-block-with-links-to-images/

No file or test is named in the issue. Start by locating the Image Compare block and the regular image block’s existing “insert from URL” flow; done when Image Compare offers that option and accepts an image URL alongside its current sources.

Written by the indexing model from the issue text.