Support annotating fields as list of primitives

[mammothb]

Description

Currently, if a model is defined as

class Hero(GraphQLQueryBaseModel):
    name: str
    friends: List[str]

calling graphql_fields crashes with

AttributeError: type object 'str' has no attribute 'model_fields'

because the _get_fields is recursively called on the element type which is a primitive and does not have the model_fields attribute.

Changes

• Check that the list element type is a custom type before recursively calling _get_fields