chore: update monitoring stack for scraping nifi metrics

[xeniape]

Part of stackabletech/nifi-operator#813. Updates the monitoring stack by enabling mTLS communication between Prometheus and NiFi to allow scraping metrics.

WIP: This needs also an update to the prometheus-community Helm Chart version once this issue is fixed. Currently the Prometheus CRD omits labels and annotations in the VolumeClaimTemplates of the additional volumes, therefore preventing secret-operator from generating the client certificates.
If the issue doesn't get fixed any time soon, we could also try to replace the Prometheus CRD with the one generated by the fix in prometheus-operator/prometheus-operator#7723 (already tested locally).

[xeniape]

Closing this in favor of #284, which includes another workaround of having an additional deployment creating the mTLS certs in Secrets, which ServiceMonitors can use.

This PR can be used later when the issue has been fixed upstream.