nvme-of discovery

[from https://pad.ceph.com/p/rbd_nvmeof_discovery_service]

NVMeOF hosts will utilize an NVMeOF discovery service to discover NVMe subsystems and the
associated transport details for how to connect. The goal of this project is to build a
generic Python library to implement the NVMeOF discovery service spec where the client of
this library (e.g. the Ceph NVMeOF management plane) can expose NVMe subsystems for
a particular host in an high-level implementation.

Background:
- NVMeOF Spec Section 5: https://nvmexpress.org/wp-content/uploads/NVMe-over-Fabrics-1.1-2019.10.22-Ratified.pdf
- Discovery client implementation: https://github.com/linux-nvme/nvme-cli/blob/master/fabrics.c

Start by reading NVMe-over-Fabrics specification Section 5 and comparing the discovery client in nvme-cli's fabrics.c. Define the library boundary and supported discovery and transport details from those references. Done means a generic Python library can expose NVMe subsystems and connection information for a host through a high-level management-plane interface.

Written by the indexing model from the issue text.