Setting libfabric environment variables per client in a process

[iziemba]

With a single process supporting multiple libfabric clients (e.g., MPI and DAOS), this raises the question how to properly set libfabric environment variables. Specifically, one set of environment variables may only be applicable to one libfabric client (example: #10526)

Has anyone else ran into this problem? For CXI provider specific env vars, we do support setting some via fi_domain get/set opts. But, this seems like a more general problem where a libfabric-level solution would be ideal.