`ForeignKeyField`s `model` attribute refer to the last subclass in mixins

Describe the bug
It's impossible to use ForeignKeyFields in mixins. model attribute gets overwritten during metadata initialization each time mixin is subclassed.

To Reproduce

from tortoise import Model, fields


class ModelMixin:
    id = fields.IntField(pk=True)
    parent = fields.ForeignKeyField("models.SomeModel", related_name="children")


class SomeModel(Model, ModelMixin):
    ...


class AnotherModel(Model, ModelMixin):
    ...


print(f'{SomeModel.parent.model_name=}')
print(f'{SomeModel.parent.model=}')
print()
print(f'{AnotherModel.parent.model_name=}')
print(f'{AnotherModel.parent.model=}')

SomeModel.parent.model_name='models.SomeModel'
SomeModel.parent.model=<class '__main__.AnotherModel'>

AnotherModel.parent.model_name='models.SomeModel'
AnotherModel.parent.model=<class '__main__.AnotherModel'>

Expected behavior
One of the following:

1. ForeignKeyFields in mixins are supported: fields refer to the correct models
2. ForeignKeyFields in mixins are not supported: exception is raised when trying to use them, docs mention this limitation

Additional context
Python 3.9.1
tortoise_orm 0.16.20

Start at the metadata initialization path for ForeignKeyField and reproduce the mixin example, checking how model and model_name are assigned for each subclass. Trace the existing field and model registration tests, then add regression coverage for the chosen outcome: correct per-model references, or a clear failure and documentation if mixin relations are unsupported.