Add Volcengine support for TTS and ASR services

- Introduced Volcengine as a new provider for both TTS and ASR services.
- Updated configuration files to include Volcengine-specific parameters such as app_id, resource_id, and uid.
- Enhanced the ASR service to support streaming mode with Volcengine's API.
- Modified existing tests to validate the integration of Volcengine services.
- Updated documentation to reflect the addition of Volcengine as a supported provider for TTS and ASR.
- Refactored service factory to accommodate Volcengine alongside existing providers.

engine/providers/tts/__init__.py

@@ -1 +1,5 @@
 """TTS providers."""
+
+from providers.tts.volcengine import VolcengineTTSService
+
+__all__ = ["VolcengineTTSService"]