add libfabric injection examples

[j-ogas]

Add Libfabric provider DSO injection support to Charliecloud.

Related to: #1429

Libfabric acts as middleware between MPI libraries and the network hardware. Fabric diversity is achieved through providers, e.g., tcp, gni, psm3, verbs, shm, etc. These providers can be compiled as built-in or loadable dynamic shared objects (*-fi.so). Loadable providers can be used by a libfabric that did not originally compile them; thus, we can add loadable provider(s), built on a separate host, to a container where they will be loaded and used by the container libfabric.

By updating our MPI examples to use Libfabric, we:

1. have a new, simpler, way to leverage Cray Gemini/Aries interconnect. We can build the gni provider on the cray system and add them to MPI containers at run-time to use the network fabric.
2. can make use of Cray's Slingshot interconnect by replacing our container libfabric with the hosts.
3. can update our MPI examples to use a Libfabric that compiled built-in providers for our fabrics of interest.