cephfs-data-scan: pgls filter for 0th file objects
Currently, when iterating over 0th objects, we are actually listing all the objects and selecting the ones we care about client side.
That's clunky for systems with big files where the 0th objects are in a small minority. Implement an OSD-side PGLS filter for this case. The filter could be something generic like a comparison of trailing characters in the object name.
If doing this after #12133, also implement a pgls filter for the files that aren't touched by forward scrub.