Allow multiple ajax sorts and changes to result size

[mfriedy]

Allow the passing of multiple columns into the order_by field of form_ajax_refs, and also allow the passing of a limit parameter, rather than always using the default.

This allows configurations such as

    form_ajax_refs = {
        "order": {
            "fields": ("country_id", "user_id", "order_date"),
            "order_by": ["user_id", "order_date"],
            "limit": 50,
        }
    }

[aminalaee]

Thanks for the PR. Can you please add some unit tests for your change?

[aminalaee]

Thanks for the PR