librbd/migration/NativeFormat: support specifying mon_host and key via spec

librbd/migration/NativeFormat: support specifying mon_host and key via spec

migration:
           -take  secret_key from spec
           -take  mon_host from the spec
           -ignore source keyring, source ceph.conf - bypass them
           -added validations of the new way schema
           -get key from the KV DB if key in spec  is actually key-ref

Fixes: https://tracker.ceph.com/issues/68177
Signed-off-by: Leonid Chernin <leonidc@il.ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 705d865ae77f4d3e80a9d3da3281ba6a824d7f29)