improve task creation and cancellation

If a FrameProcessor needs to create a task it should use
FrameProcessor.create_task() and FrameProcessor.cancel_task(). This gives
Pipecat more control over all the tasks that are created in Pipecat.

Both functions internally use the utils module: utils.create_task() and
utils.cancel_task() which should also be used outside of FrameProcessors. That
is, unless strictly necessary, we should avoid using asyncio.create_task().

examples/foundational/22b-natural-conversation-proposal.py

@@ -169,8 +169,7 @@ class OutputGate(FrameProcessor):
         self._gate_task = self.get_event_loop().create_task(self._gate_task_handler())
 
     async def _stop(self):
-        self._gate_task.cancel()
-        await self._gate_task
+        await self.cancel_task(self._gate_task)
 
     async def _gate_task_handler(self):
         while True: