[Feature] Enable Cascade Behavior for generic_relationship

[visuman]

Description
I would like to propose the addition of cascade behavior to the generic_relationship feature in SQLAlchemy-Utils. Currently, the absence of cascade options on generic relationships limits their ability to automatically propagate changes to linked tables.

Example
Consider the following example:

class User(Base):
    __tablename__ = 'users'
    id = sa.Column(sa.Integer, primary_key=True)


class Customer(Base):
    __tablename__ = 'customer'
  id = sa.Column(sa.Integer, primary_key=True)

class Event(Base):
    __tablename__ = 'event'
    id = sa.Column(sa.Integer, primary_key=True)
    # This is used to discriminate between the linked tables.
    object_type = sa.Column(sa.Unicode(255))
    # This is used to point to the primary key of the linked row.
    object_id = sa.Column(sa.Integer)
    object = generic_relationship(object_type, object_id)

# Add user
user = User()
session.add(user)
session.commit()

# Add event
event = Event()
event.object = user
session.add(event)
session.commit()

# Delete user
user = session.query(User).first() # only 1 user
sessin.delete(user)
session.commit()


# Get event and print object, object_id
event = session.query(Event).first() # This should not have returned any result if cascade-delete was supported in generic_relationships

print(event.object)
# None

print(event.object_id)
# 1

In this example, when an instance of User is deleted, I expect the associated records in the Event table to be automatically deleted as well, assuming cascade behavior is enabled for the generic_relationship.

Additional Notes:

Enabling cascade behavior on generic_relationship would enhance the usability and consistency of generic relationships in SQLAlchemy-Utils, providing a more intuitive and comprehensive solution for managing relationships between tables.

Thanks in advance