Define state for a session when calling `adk web`

** Please make sure you read the contribution guide and file the issues in the rigth place. **
[Contribution guide.](https://google.github.io/adk-docs/contributing-guide/)

**Is your feature request related to a problem? Please describe.**
Yes. I have an agent that depends on `tool_context._invocation_metadata.user_id` which when calling `adk web` always returns `"user"` which is not what I want for debugging (as I'd like it to be a real user ID).

**Describe the solution you'd like**
The ability to either by the `adk web` CLI to define what state I'd like in all my sessions, or through the UI be able to edit the entire state. Even better if each agent can define what state they depend on (or it can be inferred) and the UI can allow us to edit the specific state required for an agent.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.