feat(realtime): migrate the previous realtime models and agent back to the new version

**Background**

AgentScope has supported multiple realtime (full-duplex, low-latency) models and a `RealtimeAgent` class in previous versions. As we move toward 2.0, we plan to re-integrate and redesign this support to align with the new architecture (credential module, unified `DataBlock`, etc.).

**Changes**
A new realtime module (model base + unified events + provider implementations) and the corresponding `RealtimeAgent` class.