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/07x-interruptible-local.py

@@ -82,7 +82,7 @@ async def main():
         ),
     )
 
-    context.add_message({"role": "system", "content": "Please introduce yourself to the user."})
+    context.add_message({"role": "user", "content": "Please introduce yourself to the user."})
     await task.queue_frames([LLMRunFrame()])
 
     runner = PipelineRunner()