Finalize tool.Tool interface

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.

Start by reading the tool.Tool definition, its LLMAgent integration, and the existing constructors such as functiontool.New and loadartifacttool.New. Clarify the interfaces required for custom tool types; done means a custom type can implement tool.Tool implicitly and function correctly when passed to LLMAgent.

Written by the indexing model from the issue text.