osd: Move the test of scrubbing routines to the backend

Move enum scrub_error_type to osd_types.h
Move PG::_compare_scrub_objects to ReplicatedBackend::be_compare_scrub_objects
Move PG::_select_auth_object to ReplicatedBackend::be_select_auth_object
Move PG::_compare_scrubmaps to ReplicatedBackend::be_compare_scrubmaps

Fixes: #5997

Signed-off-by: David Zafman <david.zafman@inktank.com>