Implement Database-Backed Session Storage (DatabaseSessionService) for ADK Java

ADK Java currently provides only in-memory session storage, which is not suitable for production environments that require durable, consistent, and queryable session/state/event storage.

Projects building on ADK often need:

- Persistent session storage across restarts
- Reliable event logging and retrieval
- Concurrency-safe updates
- Support for standard relational databases
- Schema versioning for continuous delivery

## Describe the solution you'd like

Introduce a production-ready database-backed session service (`DatabaseSessionService`) that provides durable storage for sessions, events, and state.

### Key characteristics:

- **Pure JDBC implementation** with zero ORM dependencies
- **HikariCP** connection pooling for high performance
- **Flyway** migrations for schema versioning and zero-downtime deployments
- **Multi-database support**: PostgreSQL, MySQL, H2, Cloud Spanner (SQLite not supported)
- **3-tier state storage**: Separate tables for app-level, user-level, and session-level state
- **Pessimistic locking** for safe concurrent updates
- **Dialect-aware JSON handling** (JSONB for PostgreSQL, JSON for MySQL, CLOB for others)
- **Reactive API** with RxJava 3 (Single/Maybe/Completable)
- **Event filtering and pagination**
- **Comprehensive test coverage** using H2 and integration tests on actual databases (PostgreSQL, MySQL, Spanner) running in Docker

This service offers a robust, scalable alternative to the current in-memory session implementation without introducing heavy ORM dependencies.

## Describe alternatives you've considered

**Providing this as an external extension/library**
- Persistent sessions are commonly required; shipping this in-core provides a better out-of-the-box experience.

**Continuing with in-memory storage only**
- Not viable for production deployments requiring durability or concurrency guarantees.

**Using Hibernate/JPA**
- Adds significant dependency overhead and complexity; pure JDBC provides better control and performance.

## Additional context

The Python ADK includes a production-grade `DatabaseSessionService` that is widely used in real-world workloads (including GKE deployments). Adding equivalent persistent session storage to ADK Java would
provide consistency across ADK environments and reduce the need for custom user-built solutions.

An implementation is available in PR: https://github.com/google/adk-java/pull/700