.NET renaming Microsoft.AutoGen.* package

[LittleLittleCloud]

Why are these changes needed?

Update package name/structure to match what other packages does

• Microsoft.AutoGen.ServiceDefaults => Microsoft.AutoGen.Extensions.Aspire
• AIModelClientHostingExtensions => Microsoft.AutoGen.Extensions.MEAI

It also rename the corresponding folder to sub-package name

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.