suggestion: allow using NatSpec docstring instead of mandated Ellipsis in Interfaces

[fubuloubu]

Kind of borrowed from python ABCs, but I've always kind of liked that you just need some AST node that fits under an interface for @abstractmethod to work. This is nice in Python because docstrings "flow" through the inheritance heirarchy, so if you don't override the docstring on a subclass then you get a docstring "for free". I know that vyper doesn't support this rn (it could in the future), but anyways it's just a little bit annoying to mandate ... when you have a docstring and if possible it would be nice if I didn't have to

FunctionDeclarationException: function body in an interface can only be `...`!