Add a complete gRPC example

[JP-Ellis]

Description

Add an example that showcases how the gRPC plugin should be used.

Following the pattern of the new examples.

Motivation

To help facilitate the uptake of Pact Python v3, and also to assist with the needs of a current user of Pact Python.

Have you tried building it?

No response

Self-service

• I'd be willing to contribute this feature to Pact Python myself.

[github-actions]

🤖 Great news! We've labeled this issue as smartbear-supported and created a tracking ticket in PactFlow's Jira (PACT-4263). We'll keep work public and post updates here. Meanwhile, feel free to check out our docs. Thanks for your patience!