docs: add docstring for voice_cloning_key and update CHANGELOG

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Added
 
+- Added `voice_cloning_key` to `GoogleTTSService` to support custom cloned voices.
+- Added `speaking_rate` to `GoogleTTSService.InputParams` to control the speaking rate.
 - `BaseOutputTransport` now implements `write_dtmf()` by loading DTMF audio and
   sending it through the transport. This makes sending DTMF generic across all
   output transports.

src/pipecat/services/google/tts.py

@@ -523,6 +523,7 @@ class GoogleTTSService(TTSService):
             credentials: JSON string containing Google Cloud service account credentials.
             credentials_path: Path to Google Cloud service account JSON file.
             voice_id: Google TTS voice identifier (e.g., "en-US-Chirp3-HD-Charon").
+            voice_cloning_key: The voice cloning key for Chirp 3 custom voices.
             sample_rate: Audio sample rate in Hz. If None, uses default.
             params: Language configuration parameters.
             **kwargs: Additional arguments passed to parent TTSService.