AIP-72: Allow retrieving Assets via API Server & Task Context

[kaxil]

To support outlet_events & inlet_events in Context dict within the Task SDK, we will need an endpoint on the API Server which we can fetch when outlet_events & inlet_events are accessed.

airflow/airflow/models/taskinstance.py

Line 1024 in a63b652

"inlet_events": InletEventsAccessors(task.inlets, session=session),

airflow/airflow/models/taskinstance.py

Line 1019 in a63b652

"outlet_events": OutletEventAccessors(),