[AIP-31] Add "output" property in BaseOperator

[casassg]

Description

Output property should return the default XComArg for the operator (using return_value as key).

Use case / motivation

It can be used to get XComArg of an operator without needing to directly initialize XComArg explicitly.

  get_ip = SimpleHttpOperator(
      task_id='get_ip', endpoint='get', method='GET', xcom_push=True
  )

send_email = EmailOperator(
      task_id='send_email',
      to="example@example.com",
      subject='',
      html_content=get_ip.output
  )

Related Issues

Blocked by #8052