feat: relabelings support for ServiceMonitor

[pasteley]

Why is this pull request needed and what does it do?

When using Prometheus ServiceMonitor to scrape CoreDNS metrics, users may need to modify or filter labels on metrics before they are ingested into Prometheus. For example:

• Adding custom labels to identify the source cluster/environment
• Renaming labels

Example usage:

  prometheus:
    monitor:
      enabled: true
      relabelings:
        - sourceLabels: [__meta_kubernetes_pod_node_name]
          targetLabel: node

Which issues (if any) are related?

Checklist:

• I have bumped the chart version according to versioning.
• I have updated the chart changelog with all the changes that come with this pull request according to changelog.
• Any new values are backwards compatible and/or have sensible default.
• I have signed off all my commits as required by DCO.

Changes are automatically published when merged to main. They are not published on branches.

Note on DCO

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