.Net: Implement SK/AutoGen Shared Runtimes

[alliscode]

It was decided on Jan 6th that the shared runtime be located in a common repo with its own release cycle:

https://github.com/microsoft/agent-runtime

Current runtime abstractions and definitions exist as part of AutoGen packages:

https://github.com/microsoft/autogen

Initial prototyping has occurred using these existing abstractions/definitions. Once we've validated our understanding/usage of these across Process and Agents patterns we can move forward with the shared repo with expedience.

Specifically: The multi-agent chat pattern will been explored before migrating to this shared repo.

The shared runtime will consist of an Abstractions layer, and then a set of concrete runtimes that are built on this Abstractions layer:

This task covers the creation of:

• Critical repo infra
• Runtime abstractions
• Some initial subset of concrete runtimes
• Tests
• Initial build-pipeline

Once present, additional tasks include:

• More concrete runtimes
• Evolve build pipeline
• Publish pipeline

[joslat]

@alliscode / @crickman will this (https://github.com/microsoft/agent-runtime) be open source? glad to give a hand there whenever possible. :)