Store state tool should be a boolean in all agents that are provided

[kanlanc]

This is probably just me but it took me a solid minute to figure out that I need to write a store_state_tool for the agent to actually update the state beyond the output key parameter.

Ofc, not all agents need to update states and so, I would like to propose adding another boolean property to agent classes if they have the permission to update states.

This would especially be helpful to new comers instead of figuring out on their own

[Jacksunwei]

Actually session state is just for agents/tools to share information. it's system data. End user doesn't need to access it, so there is no need for ACL.

You can simply use the context from tools or callbacks to read or write the state.

You could check out our dev samples, e.g. https://github.com/google/adk-python/blob/main/contributing/samples/hello_world/agent.py#L35