Introduced a new AggregatedTextFrame Frame type that TTSTextFrame inherits from

This frame introduces an `aggregated_by` field to describe the type of text included
in the frame and allows unspoken groupings of text to be pushed through the pipeline
and treated similar to TTSTextFrames.

src/pipecat/frames/frames.py

@@ -12,6 +12,7 @@ and LLM processing.
 """
 
 from dataclasses import dataclass, field
+from enum import Enum
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -361,8 +362,32 @@ class LLMTextFrame(TextFrame):
         self.includes_inter_frame_spaces = True
 
 
+class AggregationType(str, Enum):
+    """Built-in aggregation strings."""
+
+    SENTENCE = "sentence"
+    WORD = "word"
+
+    def __str__(self):
+        return self.value
+
+
 @dataclass
-class TTSTextFrame(TextFrame):
+class AggregatedTextFrame(TextFrame):
+    """Text frame representing an aggregation of TextFrames.
+
+    This frame contains multiple TextFrames aggregated together for processing
+    or output along with a field to indicate how they are aggregated.
+
+    Parameters:
+        aggregated_by: Method used to aggregate the text frames.
+    """
+
+    aggregated_by: AggregationType | str
+
+
+@dataclass
+class TTSTextFrame(AggregatedTextFrame):
     """Text frame generated by Text-to-Speech services."""
 
     pass