Add Close method to tool.Toolset interface

[dpasiukevich]

This is a draft/idea yet.

We should add Close() error method to the tool.Toolset interface:

• consistency with adk-python source
• this allows to clean up any resources Toolset may have (e.g. session in the MCP Toolset).

[ju4nv1e1r4]

I just implemented it. This is the PR: #502