LLMAssistantAggregator: don't mark function calls as completed when receiving user image

Before, when requesting a user image from a function call we had to wait for a
random time before we could indicate the function call was done. This was to
given time to the aggregator to process the image before marking the function
call as completed.

To avoid this, we now wait for the requested image to be received by the LLM
assistant agrgegator (using an asyncio event). Then, we can successfully mark
the function call as completed.

src/pipecat/frames/frames.py

@@ -11,6 +11,7 @@ including data frames, system frames, and control frames for audio, video, text,
 and LLM processing.
 """
 
+import asyncio
 from dataclasses import dataclass, field
 from typing import (
     TYPE_CHECKING,
@@ -1218,6 +1219,7 @@ class UserImageRequestFrame(SystemFrame):
     function_name: Optional[str] = None
     tool_call_id: Optional[str] = None
     video_source: Optional[str] = None
+    request_event: Optional[asyncio.Event] = None
 
     def __str__(self):
         return f"{self.name}(user: {self.user_id}, video_source: {self.video_source}, function: {self.function_name}, request: {self.tool_call_id})"