Create PodMonitor resource to automate Prometheus monitoring

# Describe the feature you'd like to have #

Each Ceph-CSI Pod should have a PodMonitor, so that Prometheus knows how to scrape the CSI metrics.

Some considerations:
- Pods with hostnetworking (by default the node-plugins) can easily cause conflicts when listening on a local port
- ...

## Who is the end user and what is the use case where this feature will be valuable? ##

Through the Prometheus Operator it is possible to get statistics and setup alerting. This helps administrators identifying issues.

# How will we know we have a good solution? (acceptance criteria) #

Ceph-CSI metrics (from the standard Kubernetes CSI sidecars) become available in Prometheus.

# Additional context #

Suggested in ceph/ceph-csi#6287. Rook supports ServiceMonitor for the same purpose.
More details in https://prometheus-operator.dev/docs/developer/getting-started/ .