Allow extending interface types

Product

Hot Chocolate

Is your feature request related to a problem?

Related: #5974

Currently, HC allows extending all types implementing an interface, but not the interface itself.

Adding the ability to extend interfaces would greatly help domain-focused codebases where the domain types do not correspond exactly to the GraphQL types, and where one wants to keep the GraphQL layer as a separate layer on top of the domain layer. I do that in my codebases - I use explicit binding globally, and use type extensions to have full control over what goes in the GraphQL schema.

The solution you'd like

Allow extending interfaces just like we can now extend object types.

In #5974, it was mentioned that a type interceptor might be used, but I do not know how to accomplish this using a type interceptor. I just want to write a normal, friendly type extension class and have it apply to the interface itself.

Start by reviewing the existing object-type extension handling and the related discussion in #5974, then compare it with the type-interceptor approach mentioned here. The work is done when a normal type extension class can target an interface itself, with behavior consistent with existing object-type extensions and appropriate coverage in the project's tests.