.NET: Add AIAgent implementation for Github Copilot SDK

[westey-m]

There is a new Github Copilot SDK:
https://github.com/github/copilot-sdk

It has a list of usage linked from this page:
https://github.com/github/copilot-sdk/blob/main/cookbook/dotnet/README.md

We should add a new .net project Microsoft.Agents.AI.GithubCopilot. This project should provide an AIAgent implementation that uses the copilot sdk to provide copilot agentic capabilities.

The implementation should follow a similar design to A2AAgent, CopilotStudioAgent and ChatClientAgent.

All the new capabilities should have unit tests associated and a sample in Samples/GettingStarted/AgentProviders.

[moonbox3]

Same with Python? :)

[dmytrostruk]

Same with Python? :)

#3404 :)

[Meir017]

closed by #3395 ?