Add example for tool that can be invoked as a task

[the-isz]

Is your feature request related to a problem? Please describe.
I would like to implement a tool that supports task-based invocation, but I have a hard time figuring out what is needed without a basic example.

Describe the solution you'd like
There should be a simple/basic example that demonstrates how to enable task-based invocation for a tool with rmcp.
Ideally, README.md should also mention that rmcp supports task-based invocation for tools and refer to the example.

Describe alternatives you've considered
The only alternative is figuring out things myself by reading the code base :/