Update ValidationData to include metafields

[aledalgrande]

Background

Instead of using Direct API to load the validation with its metafields inside an Admin UI extension to build the configuration experience, which involves adding an extra fetch call and soon an additional app access scope to be able to read those metafields, we're simplifying the experience.

Solution

We are now exposing metafields in the type of the Admin UI extensions registration callback api parameter, following the release of the implementation that makes them available.

Checklist

• I have 🎩'd these changes
• I have updated relevant documentation