[AIP-94] airflowctl dags: add next-execution command

Summary

Add airflowctl dags next-execution --dag-id <dag_id> to airflowctl.

Implementation

• anything work with single endpoint under operation.py
• anything needs multiple endpoint or transformation goesn ctl/commands
• including unit and integration test under airflow-ctl-tests/

Acceptance criteria

• airflowctl dags next-execution --dag-id my_dag prints the next execution time.
• Returns a clear message when no upcoming run is scheduled.