Custom post types must have `'custom-fields'` support to work with Byline Manager

[mboynes]

Description of the bug

This line is the problem:

const meta = select('core/editor').getEditedPostAttribute('meta');

The meta attribute is apparently only set if the post type has 'custom-fields' support.

Steps To Reproduce

1. Register a post type with 'author' support but not 'custom-fields' support.
2. Create a new post in this post type. Note the Byline slotfill tries to render but indefinitely spins.

Additional Information

I'm not sure what the solution is here; perhaps adding custom REST API properties to supported post types to avoid having to interact with the meta attribute? I'm curious how other plugins handle this, surely Byline Manager isn't unique in hitting this problem.