Extract index key scanning from index creation task

[livius2]

Hi

This is a preparatory refactoring for a future optimizer statistics change. It extracts the relation/key scanning loop from IndexCreateTask::handler() into a reusable helper that computes index keys and delegates per-key processing to a sink/callback.

No behavior change is intended.
I added a small test:
idx_key_scanner_refactor_test.py

MSG                             regular rebuild OK
MSG                             expression rebuild OK
MSG                             partial unique rebuild OK
MSG                             unique duplicate check OK
MSG                             primary key null check OK
MSG                             foreign key bad data rejected OK
MSG                             foreign key valid data accepted OK