provide a default_per_page setting

[wolvever]

In this code https://github.com/pallets-eco/flask-sqlalchemy/blob/main/src/flask_sqlalchemy/pagination.py#L132

When per_page = 0, it will be set to 20 if error_out = False.

        if per_page < 1:
            if error_out:
                abort(404)
            else:
                per_page = 20

20 might not be the case for all scenarioes, better give it a configurable value.

[davidism]

Adding a default_per_page=20 argument to Pagination.__init__ as well as the corresponding methods makes sense. There are some other spots besides the one you linked that use that default value as well.