This moves the SQL definitions for policy and job APIs to their
separate files to improve code structure. Previously, all of these
user-visible API functions were located in the `bgw_scheduler.sql`
file, mixing internal and public functions and APIs.
To improved the structure, all API-related functions are now located
in their own distinct SQL files that have the `_api.sql` file
ending. Internal policy functions have been moved to
`policy_internal.sql`.
