rgw: provide a subcommand under radosgw-admin to list rados objects for a given bucket
The "orphans find" subcommand of radosgw-admin can be slow for a variety of reasons, particularly in that its first step is to list and record all rados objects in a pool and store them across a set of sharded omap objects.
Determining which rados objects belong to a given bucket can be non-trivial, especially in cases where a "copy object" operation is performed, that can confuse the more typical naming schemes of rados objects.
Some rgw users might want to look for orphaned objects in a more targeted and manual manner. This subcommand is designed to aid such users.