Use Gutenberg Fields Middleware to register block fields

[danielbachhuber]

As a real-world test of what we've produced, it would be a useful experiment to try and adopt Gutenberg Fields Middleware to register block fields.

I don't expect it will be easy or 100% possible at the beginning. It would be worthwhile to spend a little bit of time trying, and then report on what's useful vs. not-useful and easy vs. hard.