tools/contrib: add upmap-remapped.py utility

This script generates upmap entries for currently remapped PGs, allowing
operators to execute large cluster changes (like massive PG splitting or
adding capacity) without triggering immediate, disruptive I/O storms.
It pins PGs to their current physical locations, deferring data migration
to the background balancer.

Note: This tool is actively maintained over at
https://github.com/cernceph/ceph-scripts. We should strive to keep this
in-tree copy in sync with that repository.

Fixes: https://tracker.ceph.com/issues/78996
Signed-off-by: Dan van der Ster <dan.vanderster@clyso.com>