AIP-72: Support better type-hinting for Context dict in SDK

[kaxil]

This PR adds a Context class that is used for Type hinting of the Context dictionary. It replaces Context in the Scheduler.

part of #44481

Next PRs:

• Port remaining keys to Task SDK's Context dict
• Change Typehint in providers while keeping support for 2.x
• Remove context dict duplication in the following code:

  airflow/airflow/models/taskinstance.py

  Lines 1009 to 1051 in 1cf1d62

  	# NOTE: If you add to this dict, make sure to also update the following:
  	# * Context in airflow/utils/context.pyi
  	# * KNOWN_CONTEXT_KEYS in airflow/utils/context.py
  	# * Table in docs/apache-airflow/templates-ref.rst
  	context: dict[str, Any] = {
  	"dag": dag,
  	"dag_run": dag_run,
  	"data_interval_end": timezone.coerce_datetime(data_interval.end),
  	"data_interval_start": timezone.coerce_datetime(data_interval.start),
  	"outlet_events": OutletEventAccessors(),
  	"ds": ds,
  	"ds_nodash": ds_nodash,
  	"expanded_ti_count": expanded_ti_count,
  	"inlets": task.inlets,
  	"inlet_events": InletEventsAccessors(task.inlets, session=session),
  	"logical_date": logical_date,
  	"macros": macros,
  	"map_index_template": task.map_index_template,
  	"outlets": task.outlets,
  	"params": validated_params,
  	"prev_data_interval_start_success": get_prev_data_interval_start_success(),
  	"prev_data_interval_end_success": get_prev_data_interval_end_success(),
  	"prev_start_date_success": get_prev_start_date_success(),
  	"prev_end_date_success": get_prev_end_date_success(),
  	"run_id": task_instance.run_id,
  	"task": task,
  	"task_instance": task_instance,
  	"task_instance_key_str": f"{task.dag_id}__{task.task_id}__{ds_nodash}",
  	"test_mode": task_instance.test_mode,
  	"ti": task_instance,
  	"triggering_asset_events": lazy_object_proxy.Proxy(get_triggering_events),
  	"ts": ts,
  	"ts_nodash": ts_nodash,
  	"ts_nodash_with_tz": ts_nodash_with_tz,
  	"var": {
  	"json": VariableAccessor(deserialize_json=True),
  	"value": VariableAccessor(deserialize_json=False),
  	},
  	"conn": ConnectionAccessor(),
  	}
  	# Mypy doesn't like turning existing dicts in to a TypeDict -- and we "lie" in the type stub to say it
  	# is one, but in practice it isn't. See https://github.com/python/mypy/issues/8890
  	return Context(context) # type: ignore

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