Improve the branching strategy in the code around SQL_PAGINATION_API_SEARCH_AFTER check

[manasvinibs]

We have introduced SQL_PAGINATION_API_SEARCH_AFTER config settings to choose the approach used for pagination queries. The default value is true today which uses Point in time with search_after API to paginate the results instead of existing scroll search API. Today we have branching strategy all across the code to support both the approaches. We need to get rid of this branching checks and improve the code or get rid of scroll approach once performance testing is done.

[manasvinibs]

Minor imrpovements suggested:

1. Add a separate PIT request implementation class in the OpenSearchQUeryRequest.java - https://github.com/opensearch-project/sql/pull/2981/files/770ae4151d3e46a9fcd8246690aa20b266f69d57#diff-96f998c99595de19404d0dd5c38b71b5fb7c52160cf34a847c2785dbb05f1d03