discovery service authentication

This depends on the basic NVMe-oF discovery feature (#63). Needed for https://github.com/ceph/ceph-nvmeof/issues/119

We'd like a host to be able to authenticate the discovery service (it is what t says it is, and what the host expected to find), and vice/versa (the discovery service can confirm the host is what it claims to be).

This will be used to reveal Ceph gateway groups to only specific authenticated hosts (not just anyone who asks).

Start by reading the basic NVMe-oF discovery feature in issue #63 and the dependent request in issue #119. The work is complete when the discovery service and host authenticate each other and Ceph gateway groups are revealed only to authenticated hosts.

Written by the indexing model from the issue text.