[Feature]: PySR interface should support ClusterManager keyword arguments

[wkharold]

Feature Request

When using a ClusterManager, the PySRRegressor interface should allow the user to specify associated keyword arguments. There are a couple of use cases for this:

1. when the user wants to pass additional flags to the workload manager designated by the cluster_manager argument. For example when cluster_manager="slurm" the user may want to specify --ntasks-per-node or --cpus-per-task, etc.
2. in an Apptainer container the user needs to specify the exename and exeflags keyword arguments if cluster_manager="slurm".