Extend P4Runtime server C API to support registering a logger

We may want to add a C wrapper for DeviceMgr logging (https://github.com/p4lang/PI/blob/master/proto/frontend/PI/frontends/proto/logging.h) to the P4Runtime server C API (https://github.com/p4lang/PI/blob/master/proto/server/PI/proto/pi_server.h) so that C applications which use this API to run a P4Runtime server are able to provide a custom logger for DeviceMgr log messages.