Simplify PodTemplateSpec construction for Kueue (#151)

Change strategy for constructing the PodTemplateSpec for Kueue to
eliminate reliance on the copied Kubernetes internal defaulter for
PodSpec. Now, we just copy the subset of the PodTemplateSpec that is
necessary for Kueue's construction of the Workload object. This
eliminates most of the fields with complex defaulting behavior.

Author: dgrove-oss