Add Interactions API support to adk-java

**Description:**

Interactions API ( https://blog.google/innovation-and-ai/technology/developers-tools/interactions-api/) provides a unified interface for interacting with Gemini models and agents with stateful multi-turn conversations and server-side history management.

This feature is already available in adk-python (since v1.21.0) but is currently missing in adk-java, creating a feature parity gap between the two SDKs.

**Proposed Solution**

Add support for the Interactions API in adk-java, similar to how it's implemented in adk-python with the use_interactions_api=True parameter in the Agent configuration.

**Additional context**

- https://blog.google/innovation-and-ai/technology/developers-tools/interactions-api/
- https://developers.googleblog.com/building-agents-with-the-adk-and-the-new-interactions-api/
- https://github.com/google/adk-python/tree/main/contributing/samples/interactions_api