GraphQL Input Interface

While there seems to be work on an RFC for input unions, I think interfaces also deserve an input type for the same reason. Constructing a list of interfaces is, at the moment, quite painful.

While one could work around this by using unions, interfaces are often a better way to model a schema. The syntax should be similar to that of input unions ([RFC](https://github.com/graphql/graphql-spec/blob/master/rfcs/InputUnion.md)).