Make it clearer when endpoints modify the entity they're saving

[dsas]

What problem does this address?

When saving an entity in Gutenberg it sends the entity being saved to the server, and the server responds with what it actually saved. Gutenberg then applies the saved version to its state.

I've experienced multiple problems where a bug, both in Gutenberg and in our own code have meant that the server replies back with something different different than was requested - usually something missing. These issues are can be a pain to track down and also be rather subtle. I'd like this to be easier.

Here's a list of some example bugs which might have been easier to find and fix:

• Global Styles: unable to save block variations when KSES is active #53428
• Duotone: Custom filter colors do not work #51023
• Custom CSS not saving #46651
• Aria attributes disappear during save #39351
• Cover image cannot be added without unfiltered_html capability #2539
• Global Styles: unable to save block some variation styles when KSES is active #66799

What is your proposed solution?

It'd be helpful in diagnosing these bugs if there was a log that something was different. Ideally some kind of diff, but at the minimum a log message saying something like "Content in server response differs from request".