Add metadata and pageNotFound to createExternalDomain service

[apburnes]

Related to #3722

Changes proposed in this pull request:

• [x] Add metadata annotations to the service for createExternalDomain method
  • The following annotations to be appended to the service_instance:
    • domains, error_responses,origin, path
• [x] Add pageNotFound param to createExternalDomain method add a custom 404 page to the error_response parameter
• [ ] Add column to domain model to record error_response parameter
• [ ] Update existing external domain services with metadata annotations

security considerations

None