Make nodeSelector configurable

In Kubernetes, pods can be assigned a nodeSelector (see e.g. https://git.se.informatik.uni-kiel.de/she/theodolite/-/blob/se-cluster-config/execution/uc1-application/aggregation-deployment.yaml), which controls on which nodes a pod is scheduled. Theodolite should provide an option to set this nodeSelector.

Potentially, this issue can even be tackled more generally, by allowing for arbitrary pod adjustments.