…S backend

This commit introduces the Conversation Service module, which provides session state persistence capabilities using Alibaba Cloud TableStore (OTS). Key components include:

SessionStore: Manages session CRUD operations and cascading deletes. OTSBackend: Encapsulates OTS SDK operations for table creation and data manipulation. Data Models: Defines core data structures for sessions, events, and states. Utilities: Provides helper functions for state serialization and timestamp generation. Documentation: Includes usage examples and design documentation. Additionally, a new dependency on tablestore has been added to pyproject.toml to support the OTS operations.

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