Fix Requester API

[RashmicaG]

Some issues with the requester API are mentioned here.

1. It violates the documentation point here of "nothing openbmc specific in this repo".
2. The APIs do not actually work outside of a very specific mctp path.
3. It violates our overall architecture of not having protocol details leaking into every other repository. I have no idea, for instance, why phosphor-logging has any knowledge whatsoever of PLDM as it does today.

[RashmicaG]

If we change the API to allow for optional backends (the current mctp-demux-daemon, AF_MCTP and eventually nc-si rbt), migrate applications that use the mctp-demux to AF_MCTP, and eventually remove mctp-demux, then this address point #2?