formhandler: should support multiple sort

Summary

Currently, from the interface, user can select only column to sort on. However Gramex FormHandler supports multiple column sort by Column order.

For example, this https://learn.gramener.com/guide/formhandler/table.html#?_sort=Continent&_sort=Name

• Would show two column sort on Name, Continent
• although it doesn't highlight the columns.
• If you try to add or modify existing sort. It get's replaced by single sort.

We would need a way to support muli-column sort, if we also want to support toggle between columns for single sort.