Studio - Global fields - Completely customisable field metadata

Follow up to dependency Image title and alt tags

Phase 2:

update global fields to allow adding additional metadata fields on top of the built-in ones like alt from previous task.

Allow the agency to add a metadata field (of any reasonable type) to a specific image (or any other type) global field e.g. adding a caption field metadata field.

When defining field metadata fields, you can add helper text to describe how they should be used.

AI would be able to look at the global field definition to understand any metadata fields which were added.

Metadata fields appear for client when adding data below the main field, and show helper text if defined.