Use recursive method to correctly handle one_to_many fields

[pedro-prose]

Given:

class LabelSchema(ModelSchema):
    class Config:
        model = OrderItemLabel
        include = ['value']

class OrderItemSchema(ModelSchema):
    label: LabelSchema | None
    quantity: int

    class Config:
        model = OrderItem
        include = ['label', 'quantity']

class CustomerSchema(ModelSchema):
    class Config:
        model = Customer
        include = ['username']

class OrderSchema(ModelSchema):
    items: list[OrderItemSchema]
    customer: CustomerSchema
    
    class Config:
        model = Order
        include = ['items', 'customer']

When calling:

OrderSchema.from_django(order).dict()

Will return:

 {
    'items': [
        {'quantity': 1, 'label': None},
        {'quantity': 1, 'label': None},
        {'quantity': 1, 'label': 1}
    ],
    'customer': {'username': 'exemple@gmail.com'}
}

Note that label = 1, the ID and not the value (str) expected from LabelSchema.

With this PR change, the new output is:

{
    'items': [
        {'quantity': 1, 'label': None},
        {'quantity': 1, 'label': None},
        {'quantity': 1, 'label': {'value': 'Product 10'}}
    ],
     'customer': {'username': 'c@i.com'}}