[WIP] Universal (LLM-agnostic) context machinery to support runtime LLM switching.

- Add a "universal" alias for `OpenAILLMContextAssistantTimestampFrame`: `LLMContextAssistantTimestampFrame`

src/pipecat/frames/frames.py

@@ -403,6 +403,9 @@ class OpenAILLMContextAssistantTimestampFrame(DataFrame):
 
     timestamp: str
 
+# A more universal (LLM-agnostic) name for 
+# OpenAILLMContextAssistantTimestampFrame, matching LLMContext
+LLMContextAssistantTimestampFrame = OpenAILLMContextAssistantTimestampFrame
 
 @dataclass
 class TranscriptionMessage: