Keep same nullable context as implementation

[fjmorel]

If possible, it would great if the nullable context (off, annotations, on) could be set per item on the interface to match whatever has been set on the implementation with #nullable enable and the like.

Right now, if I set something to a nullable reference type, I get a build warning.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable'