fetch_related() should allow a manager to be passed as an arg.

[markmiscavage]

Is your feature request related to a problem? Please describe.
I have a model where a FK related object does not show in queries because the related object isn't in the query results from the default model manager. It would be cleaner and faster to be able to pass in a manager or some other argument to allow fetch_related to return that object.

example:
class GroupOneManager(Manager):
def get_queryset(self):
return (
super()
.get_queryset()
.filter(group='One')
)

class User(BaseModel):
id = fields.CharField(pk=True, max_length=255)
group = fields.CharField(max_length=255)
all_objects = Manager()

class meta: 
    manager = GroupOneManager()

class Controls(BaseModel):
user = fields.ForeignKeyField(User)

user = await User.get(group='Two').first()
await Controls(user=user).save()
control = await Controls.all.first()
await control.fetch_related('user')
print(control.user) # Will be None because user is in group='Two' which isn't part of default manager

Describe the solution you'd like
await object.fetch_related('user', manager=GroupModelManager)
of
await object.fetch_related('user', exclude_default_manger=True)

Describe alternatives you've considered
The alternative is to do a separate lookup of the object which isn't as clean or pythonic.