Make SortField immutable

### Description

`Sort` objects are shallowly immutable, but their inner `SortField` implementations are not, which could cause hard-to-track down bugs. We should make `SortField` immutable as well.

The `field`, `type` and `reverse` fields can be made final with no change in API. Missing values are trickier, as they are currently mutable via a `setMissingValue` method. My suggestion would be to deprecate this method in 10.4, and add constructors that explicitly take missing values (which could also enhance type safety); and then remove the method in 11.0.