Fix asyncio.create_task() calls

[drew2a]

This PR fixes #7299

• shutdown method has been added to the RestEndpoint base class
• List[Task] has been added as a result value of the AsyncGroup.add() call
• Calls of asyncio.create_task have been fixed

[kozlovsky]

Overall looks good. I think I found some other places where asyncio.create_task is used dangerously:

• In Service._graceful_shutdown, file hidden_peer_discovery.py:

    def _graceful_shutdown(self):
        task = asyncio.create_task(self.on_tribler_shutdown())
        task.add_done_callback(lambda result: TinyTriblerService._graceful_shutdown(self))

the variable is created, but its lifetime is shorter than the lifetime of the task.

• In Service._graceful_shutdown, file speed_test_exit.py - the same.

[drew2a]

@kozlovsky thank you for your review. Two calls of Service._graceful_shutdown were fixed.