[VSCode] annotation the backend models/operations created from template and interfaces during authoring.

[chunyu3]

When define models/operations from Azure template and interfaces, TypeSpec provide annotation to show the backend/real definition after the template parsing.

May need tsp/compiler support.

[RodgeFu]

We should be able to show the eventual signature of the operation/model which can help end user to understand what's the result from his typespec definition especially when he has a complex template inherit hierarchy. This is a common feature, not specific for 1st party.