We should allow queries to support sorting on a computed property. 

This type of advanced sorting would enable use cases like this:

• {{"location geoWithin Geospatial("polygon", [...] SORT(distance(location, GeoPoint(x,y)))" }} // sort by distance function of geospatial point
• "price - discount < 10 SORT(price - discount)" // sort by numeric computation between properties
• "name CONTAINS[c] 'on' SORT(name.@size)" // sort on length of string
• "TRUEPREDICATE SORT(myList.@size)" // sort on size of list property
• "TRUEPREDICATE SORT(myMixedProp.@type ASC, myMixedProp ASC)" // sort on type of mixed property (tracked https://github.com/realm/realm-core/issues/4586)