Adding backcompat shim for BaseNotifier

[amoghrajesh]

Trying to import BaseNotifier from the older path in core reported in

(link_name, task.get_extra_links(ti, link_name)) for link_name in task.extra_links
  File "/opt/airflow/airflow-core/src/airflow/models/mappedoperator.py", line 177, in get_extra_links
    return link.get_link(self.unmap(None), ti_key=ti.key)
  File "/opt/airflow/task-sdk/src/airflow/sdk/definitions/mappedoperator.py", line 772, in unmap
    SerializedBaseOperator.populate_operator(op, self.operator_class)
  File "/opt/airflow/airflow-core/src/airflow/serialization/serialized_objects.py", line 1348, in populate_operator
    and operator.__module__ == encoded_op["_task_module"]
KeyError: '_task_module'

We should maintain backcompat to allow seamless migration for end users.

Also interestingly the shorthand from airflow.sdk import BaseNotifier didn;t work. Fixed it in the PR too

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[kaxil]

Pre-approving since it is a simple change

[github-actions]

Backport successfully created: v3-0-test

Status	Branch	Result
✅	v3-0-test