mgmt, provide default method for fluent interfaces

[weidongxu-microsoft]

In this case, we should generate new interface methods with default. One option is to return null, which may not be valid response but it would be better to throw UnsupportedOperationException with clear message. Throwing exception will also work for operations that return void and indicate to the user something didn't work as expected.

For generated libs, we probably don't want to bother comparing it with prior version to see which method was added to interface. It likely easier to just generate all interface with a default method, which throws UnsupportedOperationException.