Language Generation CustomFunction Sample should show how to do async work.

[johnataylor]

Language Generation uses Adaptive Expressions that can be extended with custom functions.

This is illustrated by this sample https://github.com/microsoft/BotBuilder-Samples/tree/master/samples/csharp_dotnetcore/language-generation/20.extending-with-custom-functions

However, such functions cannot be async. "This is actually by design since adaptive expressions are used to evaluate triggers. The thought was you should use actions to populate memory that you then can use in .lg."

This sample should be extended to illustrate this technique.