introduce heartbeat frames

src/pipecat/frames/frames.py

@@ -424,6 +424,16 @@ class FatalErrorFrame(ErrorFrame):
     fatal: bool = field(default=True, init=False)
 
 
+@dataclass
+class HeartbeatFrame(SystemFrame):
+    """This frame is used by the pipeline task as a mechanism to know if the
+    pipeline is running properly.
+
+    """
+
+    timestamp: int
+
+
 @dataclass
 class EndTaskFrame(SystemFrame):
     """This is used to notify the pipeline task that the pipeline should be