Finalize tool.Tool interface

[dpasiukevich]

Currently is serves as a placeholder type to pass tools to LLMAgent.

It's not possible to implement tool.Tool directly with a custom type (technically possible, but this tool won't work). The only way right now to create tool -- using adk-provided constructors (e.g. functiontool.New, loadartifacttool.New).

We need to finalize tool.Tool interface (and maybe even expose any extra interfaces if needed), to allow standard implicit interface implementation of a tool by custom types.

[shanto12]

This is an important interface design consideration. Making tool.Tool interface more flexible would definitely help with custom tool implementations.

A few thoughts:

• Exposing the interface could allow for better composition patterns and reduce boilerplate
• Consider adding helper methods or validation utilities to make custom tool creation easier
• Documentation with clear examples of implementing custom tools would be valuable

I'd be happy to help test any proposed changes or contribute to documentation once the interface is finalized.

[verdverm]

One question I have is if tools can have access to the services like ArtifactService. Will the new interface allow us to implement a custom tool like read_file the loads a file from the user device to the artifacts so our agents can access it?