Enable gateway discovery service to act as a centralized discovery controller

An NVMe centralized discovery controller (CDC) serves the same purpose as a top-level discovery server (https://github.com/ceph/ceph-nvmeof/issues/71), but improves on that by eliminating the need for every host to completely enumerate all the discovery controllers on the network to find the namespaces they can/must attach to.

To be used with many Ceph clusters would require #72.

This feature is just for the ability of the gateway discovery service to act as a CDC. As much as anything this ticket is to distinguish CDC functionality from #71

Centralized Discovery Controllers are defined in TP8010a. See the ZIP file of ratified NVMe 2.0 TPs on [nvmexpress.org](https://nvmexpress.org/wp-content/uploads/NVM-Express-2.0-Ratified-TPs_20230111.zip) for the text of TP8010a.

Start by reading the TP8010a specification and the related issues #71 and #72 to define how centralized discovery controller behavior differs from top-level discovery. The work is complete when the gateway discovery service can act as a CDC for the described use case, but this issue does not name implementation files or tests.

Written by the indexing model from the issue text.