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