SER-13233 Add support for sorting on multiple columns using CollectinOptions sortby.

[YouWillNeverWalkAl1]

Added support for specifying multiple sort by columns in the CollectionOptions sortBy string separated by commas. Each column needs to be defined in allowedSortColumns in order to be used in the sort. The sortBy string could e.g. contain the string "name asc, id desc" which would result in the corresponding sortby clause.