Rewrite request bodies from patch.yml using Spectacular

Add a serializer or use inline_serializer and apply it using extend_schema(request=TheSerializer).

• AddSelection (required array of elements ids)
• RemoveSelection (optional request body with an element id)
  • Check using the API client that body= is not required
  • Fallback if body cannot be made optional: just make id optional (body={} will be required)