Return a list of Lucene SortFields to be used for sorting search results.
Using this method allows the field to specify the most performant way to perform a search. If a field can be
sorted directly using the term in the index then this should just return a singleton list with the sort field.
return Collections.singletonList(new SortField(fieldName, sortOrder));
The default implementation builds this using the FieldComparatorSource returned by getSortComparatorSource()
If you implement this method there is no need to implement getSortComparatorSource()
.