Bug 24700: Avoid validating REST API spec multiple times

Bug 24700: Avoid validating REST API spec multiple times

This patch changes the spec loading so that it is fully loaded and merged with any plugin routes before validation. The individual parts are separately validated only if the resulting complete spec is invalid. Note: the OpenAPI plugin validates the spec given to it, so normally no manual validation is necessary.

Test plan:
1. prove -v t/db_dependent/Koha/REST/*
2. prove -v t/db_dependent/api/v1/*

Sponsored-by: The National Library of Finland
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>