introduce FrameProcessor single async push frame task

Pipecat has a pipeline-based architecture. The pipeline consists of frame
processors linked to each other. The elements travelling across the pipeline are
called frames.

To have a deterministic behavior the frames travelling through the pipeline are
awlays ordered, except system frames which are out-of-band frames.

To achieve ordering each frame processor only outputs frames from a single
task. This single task is internally created in each FrameProcessor and the
developer doesn't need to know much about it, except conceptually.

Having a single output task avoids problems such as audio overlapping.

examples/foundational/17-detect-user-idle.py

@@ -70,7 +70,7 @@ async def main():
         async def user_idle_callback(user_idle: UserIdleProcessor):
             messages.append(
                 {"role": "system", "content": "Ask the user if they are still there and try to prompt for some input, but be short."})
-            await user_idle.queue_frame(LLMMessagesFrame(messages))
+            await user_idle.push_frame(LLMMessagesFrame(messages))
 
         user_idle = UserIdleProcessor(callback=user_idle_callback, timeout=5.0)