Add database session service

[kalenkevich]

Description

This PR introduces the DatabaseSessionService, an implementation of the session service that uses a SQL database for persistence. This ensures that session state, user state, and application state are durable and can be shared across multiple instances of the application.

Key Changes

• New Service: DatabaseSessionService extending BaseSessionService.
• ORM Integration: Utilizes Sequelize to interact with the database, providing a type-safe and flexible data layer.
• Data Models: Defines the following models for structured data storage:
  • StorageMetadata: For schema versioning and internal metadata.
  • StorageAppState: Stores application-level state (keyed by appName).
  • StorageUserState: Stores user-level state (keyed by appName + userId).
  • StorageSession: Stores session-specific state (keyed by sessionId).
  • StorageEvent: Stores events associated with sessions.

Supported Databases

Thanks to Sequelize, this service supports a wide range of SQL dialects. The implementation checks connection strings against the following supported protocols:

• PostgreSQL
• SQLite
• MySQL
• MariaDB
• MSSQL

Implementation Details

• Ensures Database prepared: Created database/tables in case they does not exists (Follows the Python ADK implementation).
• Schema Validation: Includes a validateSchemaVersion check to ensure the database schema is compatible with the Python ADK DB session service.
• State Management: Implements the standard ADK state merging logic, combining App, User, and Session states into a unified view for the agent.
• Parity: Designed to maintain parity with the Python ADK's database session implementation. Stores the events in the snake_case format in the DB.