Global Session Storage

**Description**:
Currently, Google ADK provides session storage that is scoped only to individual sessions. This means that when multiple users interact with the same ADK agent tool (for example, a test case generation tool built for our company), each user’s context is isolated and not shared.
**Problem:**
In our use case, test cases for different features can be interrelated, and having access to a broader, company-wide context would significantly improve the quality and relevance of generated test cases. For example, if a user provides context for a particular feature, it would be beneficial for the LLM to be aware of related features, previously generated test cases, and company-specific requirements—even if those were provided in a different session or by a different user.
**Request:**
Please add support for a global or shared context storage mechanism in Google ADK, in addition to the current session-scoped storage. This would allow:

1. Storing and retrieving context that is accessible across all sessions and users for a given agent or tool.
2. Enabling the LLM to leverage company-wide knowledge, previously generated artifacts, and inter-feature relationships.
3. Improving the ability to generate more comprehensive, consistent, and context-aware outputs (such as test cases) that reflect the broader organizational context.

**Example Scenario:**

1. User A generates test cases for Feature X and provides detailed company context.
2. User B, in a different session, generates test cases for Feature Y, which is related to Feature X.
3. With global context storage, the LLM can reference the context and test cases from Feature X when generating cases for Feature Y, leading to better coverage and consistency.

**Benefits:**

1. Enhanced LLM understanding of company-specific requirements and inter-feature dependencies.
2. More robust and interconnected outputs for complex, enterprise-scale use cases.
3. Reduced duplication of context and improved collaboration across teams.