Move system_instruction into LLMSettings

Add `system_instruction` field to `LLMSettings` so it is runtime-updatable via settings.
For Google (GoogleLLMService, GoogleVertexLLMService), deprecate the init-time arg since it was already shipped. For Anthropic, AWS Bedrock, and OpenAI, remove the init-time arg entirely since it was never shipped.

Still need to handle realtime services (OpenAI Realtime, Grok Realtime, Gemini Live).

src/pipecat/services/cerebras/llm.py

@@ -111,4 +111,12 @@ class CerebrasLLMService(OpenAILLMService):
         params.update(params_from_context)
 
         params.update(self._settings.extra)
+
+        # Prepend system instruction if set
+        if self._settings.system_instruction:
+            messages = params.get("messages", [])
+            params["messages"] = [
+                {"role": "system", "content": self._settings.system_instruction}
+            ] + messages
+
         return params