Added an invoke command example provider

[devkelley]

Motivation and Context

This PR is needed to provide a simple example of an invoke intent implementation on the provider side.

Description

This code change adds a simple example of how a provider would be set up to handle an invoke call through chariott. It also provides a readme file that gives some sample grpcurl commands to test the invoke call.

PR COMMIT MESSAGE

docs: example code showing simple invoke intent implementation