Image Block: Add Block Bindings support for caption attribute

[Bernhard Reiter]

The Image block's caption attribute is a bit different from attributes previosuly supported by Block Bindings in that if it is present, a <figcaption> HTML element is saved as part of the HTML markup (which isn't there if the attribute is empty).

This behavior should be mimicked as closely as possible when the attribute is bound to a Block Bindings source instead. This can be partly done by including the <figcaption> in the saved HTML if the attribute is bound, and to have the <figcaption> element's inner HTML replaced by the value provided by the Block Bindings source upon render.

If the Block Bindings source returns an empty value for the caption, we might want to remove the <figcaption> element before rendering.

Supersedes ​https://github.com/WordPress/wordpress-develop/pull/9702 for reasons stated here.

TODO:

• ​x Carry over [additional test coverage from GB], see ​this convo.
• [x] Need to file a new Trac ticket -- https://core.trac.wordpress.org/ticket/63919 is no longer pertinent.
• ​https://github.com/WordPress/gutenberg/pull/71483/files#diff-749aec929bf6ec005ace1d322816dbb1e5d3b8bdfb87d2d44609cdbe744dd7e1R1 Also might need to update [this back-compat changelog in GB] to point to this PR.

Trac ticket: https://core.trac.wordpress.org/ticket/64031

[Bernhard Reiter]

In 60798:

Block Bindings: Support Image block's caption attribute.

This is now possible thank to the logic added in [60684].

Props bernhard-reiter, mukesh27.
Fixes #64031.