search: add support for custom string sort ordering

[Vishhh03]

Fixes #2223

The custom sort func within SearchRequest only sorts the value after the collector picks up during phase 1, ie. Custom sort function only runs in phase 2 after the data has been sorted normally.

1. Added an Interface that SearchSort can implement
2. Added a custom function field to search.SortField
3. Implement the custom function interface on SortField which is json excluded for serialization
4. Update sortOrder.Compare to conditionally use the custom function if implemented
5. Added a new unit test TestSortOrderCustomCompare to existing search/sort_test.go file to verify the custom sort functionality