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/43-heygen-transport.py

@@ -100,7 +100,7 @@ async def main():
             # Kick off the conversation.
             context.add_message(
                 {
-                    "role": "system",
+                    "role": "user",
                     "content": "Start by saying 'Hello' and then a short greeting.",
                 }
             )