[.NET] Add happy path test for in-memory agent && Simplify HelloAgent example && some clean-up in extension APIs

[LittleLittleCloud]

Why are these changes needed?

This PR does the following work

• Add happy-path test for in-memory use case
• Improve the existing extension APIs (essentially replacing WebApplicationBuilder -> IHostApplicationBuilder)
• disable GRPC sample in HelloAgent sample

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[rysweet]

I like this. did you try this with grpc as well to make sure it still works? it conflicts with some changes Reuben was proposing that used the .Web SDK instead of .SDK. the changes to HelloAgents I am less thrilled about mainly because I liked how the previous version did everything in one shot. maybe we could have both?

I haven't had time to try and run this yet and make sure it still also works with gprc. If you have done that let me know and I'll go ahead and merge.