Add support for @declared_attr sqlalchemy decorator

[bitphage]

SQLAlchemy has @declared_attr decorator which adds declarative fields for "base" models. It would be nice to allow to keep @declared_attr methods close to class attributes like this:

class Foo:
    field = Column(String(45), nullable=False)
    other_field = Column(String(45), nullable=False)
    more_field = Column(String(45), nullable=False)

    @declared_attr
    def client_id(cls):
        return Column(String(256), ForeignKey("UserModel.client_id"), nullable=False)

    # other stuff
    @property
    def timestamp(self) -> Timestamp_ms:
        return self.last_update_timestamp

https://docs.sqlalchemy.org/en/13/orm/extensions/declarative/api.html#sqlalchemy.ext.declarative.declared_attr