✨Add sa_foreign_key_args and sa_foreign_key_kwargs arguments to Field…

[earshinov]

…to let the user define additional sqlalchemy.orm.ForeignKey attributes, such as ondelete and onupdate, for foreign keys defined in a base model.

[YuriiMotov]

@earshinov, thanks for your interest and efforts!

IMO, the option with sa_column_args=[ForeignKey(...)] looks better than with sa_foreign_key_kwargs={} - in first case we have type hints and it's more readable.

As I understand, the problem is that in case of inheritance the same ForeignKey instance is being re-used in several models and it causes that exception.
What if we approach to solve this in a different way - use copy of ForeignKey object instead of that object itself?
I mean in get_column_from_field extract ForeignKey from sa_column_args (if exists) and pass its copy instead

[earshinov]

@YuriiMotov , thanks for a useful suggestion. Indeed, the approach you described is working and provides more type safety. I updated the PR.

[earshinov]

Docs will need to be updated, too, to make the users aware that foreign_key can now be given as a sqlalchemy.schema.ForeignKey instance instead of a string. I am not sure what would be the best place to mention this.