[Feature] Allow users to specify custom spawners #942
Description
At the moment the spawner module provides four different interfaces to start a new process, using a subprocess, mpiexec, srun or flux run. But different clusters might have different commands to trigger MPI parallel runs or in principle it would also be possible to add additional computing resources from a cloud provider to speed up an application running on your laptop. Either way it would be great to add the option to provide custom spawners.