turns: add external user and bot turn start strategies

External strategies are strategies where the logic for user turn start and turn
end come from a different processors (e.g. an STT).

src/pipecat/turns/bot/external_bot_turn_start_strategy.py

@@ -29,7 +29,7 @@ class ExternalBotTurnStartStrategy(BaseBotTurnStartStrategy):
 
     """
 
-    def __init__(self, *, timeout: float = 0.5):
+    def __init__(self, *, timeout: float = 0.2):
         """Initialize the transcription-based bot turn start strategy.
 
         Args: