rgw: add rgw-gap-list tool

rgw: add rgw-gap-list tool

Due to a prior bug (pr: 38228) tail rados objects of some RGW objects
could have been incorrectly deleted. This tool is designed to look for
such cases. It essentially does the opposite of rgw-orphan-list,
looking for rados objects that RGW expects to be there, but which are
not to be found.

IMPORTANT: This is very experimental at this point in time, and any
"results" produced should be verified by other means.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
Signed-off-by: Michael Kidd <linuxkidd@gmail.com>