feat(asr): add asr module to agentscope library

Background AgentScope lacks a native ASR abstraction. A dedicated ASR module is required as the foundation for the cascaded realtime pipeline (ASR → LLM → TTS).

Changes

• Add a new asr module under src/agentscope
• Add a base ASR model class

  Prior to implementation, survey existing ASR API providers — similar to the tts module, providers may differ significantly in patterns (e.g., streaming vs. batch transcription)

• Provide one reference implementation, leaving additional provider integrations to follow-up PRs to keep this PR focused on the abstraction layer