In worker Operator

[luiztauffer]

This PR takes the necessary steps towards Fix #49

• [x] new column piece_execution_mode for PieceRepository table
• [ ] make migrations for database model changes
• [ ] add rest endpoints for getting, creating, deleting and editing Airflow Connections
• [ ] add DominoBackendRestClient.check_create_airflow_connection() function
• [ ] DominoWorkerOperator
  • [x] activate DominoWorkerOperator when execution_mode==worker
  • [ ] load Piece code from tar file (or similar method), the Piece should require no installation in the worker environment
  • [ ] get secrets with DominoBackendRestClient
  • [ ] get conn_id with DominoBackendRestClient