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/21-tavus-transport.py

@@ -97,7 +97,7 @@ async def main():
             # Kick off the conversation.
             context.add_message(
                 {
-                    "role": "system",
+                    "role": "user",
                     "content": "Start by greeting the user and ask how you can help.",
                 }
             )