Implement DashScope ASR provider and enhance ASR service architecture

- Added DashScope ASR service implementation for real-time streaming.
- Updated ASR provider logic to support DashScope alongside existing providers.
- Enhanced runtime metadata resolution to include DashScope as a valid ASR provider.
- Modified configuration files and documentation to reflect the addition of DashScope.
- Introduced tests to validate DashScope integration and ASR service behavior.
- Refactored ASR service factory to accommodate new provider options and modes.

engine/providers/asr/__init__.py

@@ -1 +1,13 @@
 """ASR providers."""
+
+from providers.asr.buffered import BufferedASRService, MockASRService
+from providers.asr.dashscope import DashScopeRealtimeASRService
+from providers.asr.openai_compatible import OpenAICompatibleASRService, SiliconFlowASRService
+
+__all__ = [
+    "BufferedASRService",
+    "MockASRService",
+    "DashScopeRealtimeASRService",
+    "OpenAICompatibleASRService",
+    "SiliconFlowASRService",
+]