Support symmetrical ManyToMany relationships

Django ORM has support for what they call symmetrical relationships, that is a ManyToMany relationship on self. (see here https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ManyToManyField.symmetrical)

For example, if you want a ManyToMany relation for friends and you want the logic to be "if I am your friend, then you are my friend", you would use a symmetrical relation. That way, there is no related attribute generated on the other class, and for checking the friend status, you only have to look up one field instead of both.

Example

class User(models.Model):
  name = fields.TextField()
  friends = fields.ManyToMany("models.User", symmetrical=True)

>>> u1 = await User.create("Bob")
>>> u2 = await User.create("Alice")
>>> await u1.friends.add(u2)
>>> await u1.friends.filter(id=u2.id).exists()
True
>>> # bi-directional
>>> await u2.friends.filter(id=u1.id).exists()
True

I can try to contribute this, but I want the confirmation from a maintainer first

Start by locating Tortoise ORM's ManyToMany implementation and its relationship tests, then compare the requested self-referential behavior with Django's symmetrical option. Done means a self-referential ManyToMany relationship can be declared with symmetrical behavior, adding a relation through one instance makes it visible from the other, and the related attribute behavior matches the issue's example.