Update foundational examples to use "user" role

Use system_instruction on LLM service constructors instead of adding
system messages to LLMContext. Messages added to context now use
"user" role.

examples/foundational/04a-transports-daily.py

@@ -93,7 +93,7 @@ async def main():
             await transport.capture_participant_transcription(participant["id"])
             # Kick off the conversation.
             context.add_message(
-                {"role": "system", "content": "Please introduce yourself to the user."}
+                {"role": "user", "content": "Please introduce yourself to the user."}
             )
             await task.queue_frames([LLMRunFrame()])