Add core-io support for FTS keyset pagination

Suggested release note: N/A

As a prerequisite for migrating Kotlin to CoreSearchOps, add support for keyset pagination.

https://docs.couchbase.com/server/current/fts/fts-pagination.html#search_after-search_before

CoreSearchOptions gets two new options: searchBefore and searchAfter. The argument is a CoreSearchKey obtained from a CoreSearchRow.