Feat: Adding the option of importing interfaces from strings

[Hercilio1]

I want to add new interfaces to a Page model, but the problem is that those interfaces import model files to collect different sets of data from them.

The issue is that graphql_interfaces = [CustomInterface] requires importing CustomInterface inside the model file, which causes the error AppRegistryNotReady("Models aren't loaded yet.").

The load_type_fields runs at the right time, but I can’t define graphql_interfaces without importing the interface in the models file.

The fix would be something like graphql_interfaces = ["apps.custom.graphql.object_types.CustomInterface"], so I implemented it this way.

[zerolab]

Thank you. Can you add a test or two?

[Hercilio1]

@zerolab , done :)