Changes to POST /applications/:application_id/entitlements

added subschema #1: GUILD subschema #2: USER to the owner_type request property oneOf list

the response property ends_at became required for the status 200

the response property starts_at became required for the status 200

added the optional property parent_id to the response with the 200 status

added the optional property gifter_user_id to the response with the 200 status