Post Type Schemas: add internal fields and use in appropriate abilities

[priethor]

What

Adds a schema for internal fields that SCF uses for its entities and applies it to Post Type abilities's output schemas where required.

Why

• Output schemas for CREATE/UPDATE/DUPLICATE/IMPORT were missing internal fields (ID, _valid, local) that are actually returned.
• It applies to Post Types, but it will be reused for other entities
• Importing also supports the ID to overwrite an existing Post Type

Testing Instructions

1. Run composer test:php to verify schema validation tests pass
2. Run post type abilities and verify they now don't return an error