Support different service and container ports

[abaguas]

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

This PR allows exposing coreDNS on a service port different from the ports exposed by the container.
We want to expose coreDNS publicly. To increase security we would like to open a container port above 1024 to not add the NET_BIND_SERVICE capability, while still exposing the service on port 53.

The change adds an optional servers.servicePort key that can be used to set a service port. If this value is not set, the value of the servers.port is used both as service and container port (old behavior). Thus, the change is fully backwards compatible.

helm package . && helm template coredns coredns-1.33.0.tgz -f values.yaml > manifests2.yaml
...
diff manifests.yaml manifests2.yaml                                                        
111,112c111,112
<   - {"name":"udp-53","port":53,"protocol":"UDP"}
<   - {"name":"tcp-53","port":53,"protocol":"TCP"}
---
>   - {"name":"udp-53","port":53,"protocol":"UDP","targetPort":53}
>   - {"name":"tcp-53","port":53,"protocol":"TCP","targetPort":53}

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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[hagaibarel]

Small nit

[hagaibarel]

Thanks for the PR!