.NET runtime interface to match python - add registration to interface and InMemoryRuntime

[rysweet]

What feature would you like to be added?

add RegisterAgentType to the agent runtime (its already in the backend server, but in memory we don't need it because we just load the agents from relection.

Why is this needed?

In .NET when the agents are all in one process we can use reflection to register agents and technically don't need registration. But we desire to match with the python runtime interface so I'm adding it. this may also enable scenarios like the one @colombod has proposed in which the in memory runtime could at some point self-federate into a larger agent system with distributed runtimes.