Handle clearing XCom in run Execution endpoint

[kaxil]

We should clear XCom value when task starts execution in the Execution API server. This is how it is done now:

airflow/airflow/models/taskinstance.py

Lines 2846 to 2848 in f03b1d4

	# Don't clear Xcom until the task is certain to execute, and check if we are resuming from deferral.
	if not self.next_method:
	self.clear_xcom_data()

Ideally, this can be done in the run endpoint. We need to ensure we don't clear it for Deferrable tasks.

Port the necessary tests from current execution model to the API server