introduce synchronous and asynchronous frame processors

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
should always be ordered, except system frames which are out-of-band frames. To
achieve that, each frame processor should only output frames from a single task.

There are synchronous and asynchronous frame processors. The synchronous
processors push output frames from the same task that they receive input frames,
and therefore only pushing frames from one task. Asynchrnous frame processors
can have internal tasks to perform things asynchrnously (e.g. receiving data
from a websocket) but they also have a single task where they push frames from.

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)