.Net: Update code examples about providing function return types to LLM on Microsoft Learn

Update this section: https://learn.microsoft.com/en-us/semantic-kernel/concepts/plugins/adding-native-plugins?pivots=programming-language-csharp#providing-functions-return-type-schema-to-llm

Use code samples from this file to better demonstrate the idea of providing function return type info to the LLM: https://github.com/microsoft/semantic-kernel/blob/main/dotnet/samples/Concepts/FunctionCalling/FunctionCalling_ReturnMetadata.cs