cephfs admin: add quotaSizePlaceholder type for JSON unmarshaling
cephfs admin: add quotaSizePlaceholder type for JSON unmarshaling

Add a new quotaSizePlaceholder type which is a non-interface type that
can unmarshal JSON with a varying type into one of our types that meets
the QuotaSize interface.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph