Ensure interface nullable types can be implemented by non-nullable types

[maartenvanvliet]

Fixes #987

Two cases are added to the schema:

• a simple nullable interface -> non-nullable implementor.
• a nullable list -> with implementors list_of(non_null(type)) and
  non_null(list_of(non_null(type))).

From my reading of the spec these should all validate.

https://spec.graphql.org/draft/#IsValidImplementationFieldType()

I did find that it is not validated whether the object implements
the field's arguments as specified by the interface. Also that the object
does not implement extra non-nullable arguments that are not specified in the interface.

From the spec

field must include an argument of the same name for every argument defined in implementedField.

    That named argument on field must accept the same type (invariant) as that named argument on implementedField.

field may include additional arguments not defined in implementedField, but any additional argument must not be required, e.g. must not be of a non‐nullable type.

I'll create an issue for this.

[benwilson512]

Great, thanks! Good point about arguments, definitely an area we should improve.