1. Proposed title of this feature request
MonitoringOperator should support affinity options for nodePlacement

2. What is the nature and description of the request?
MonitoringOperator supports specifying nodePlacement options (nodeSelectors and tolerations). It does not currently support the more flexible podAffinity and nodeAffinity options.

3. Why does the customer need this? (List the business requirements here)
The more flexible podAffinity and nodeAffinity options would allow for conditions like preferredDuringSchedulingIgnoredDuringExecution, so that infrastructure nodes could be preferred, but if not available, the monitoring stack pods will still schedule. With how critical the monitoring pods are, it's preferred that it be allowed to schedule in many scenarios, even if there are preferences for where it should go if there is capacity.

4. List any affected packages or components.
https://github.com/openshift/cluster-monitoring-operator/blob/eec0457c7449685e8291f6e4b575e25bc3ad8a98/assets/prometheus-k8s/prometheus.yaml#L10-L22