frames: add LLMTextFrame and TTSTextFrame

This is to distinguish what type of service has generated the TextFrames.

src/pipecat/frames/frames.py

@@ -177,6 +177,20 @@ class TextFrame(DataFrame):
         return f"{self.name}(pts: {pts}, text: [{self.text}])"
 
 
+@dataclass
+class LLMTextFrame(TextFrame):
+    """A text frame generated by LLM services."""
+
+    pass
+
+
+@dataclass
+class TTSTextFrame(TextFrame):
+    """A text frame generated by TTS services."""
+
+    pass
+
+
 @dataclass
 class TranscriptionFrame(TextFrame):
     """A text frame with transcription-specific data. Will be placed in the