Allow setting imagePullSecret for coredns

[jhwagner]

Is your feature request related to a problem?

When using the default coredns deployment, the image pull secret is not configurable. This becomes an issue when using private image registries, for example in many enterprise settings where we might use a private registry as a pull through cache or for other security concerns. Currently, the only way to address this is to override the entire coredns manifest config in controlplane.coredns.overwriteManifests, which is a bit cumbersome as it requires us to redefine all of the manifests just to add a single field.

This seems to have been brought up before, e.g. in #488, which was marked resolved and the suggestion was to use serviceAccount.imagePullSecrets (which looks to now be advanced.serviceAccount.imagePullSecrets and advanced.workloadServiceAccount.imagePullSecrets but neither work). I'm not sure if this used to work and is just a regression but that field does not set the image pull secret on the coredns serviceaccount or deployment.

Which solution do you suggest?

Similar to previous issue, add imagePullSecret parameter to coredns values, making the serviceaccount or deployment configurable. Something like controlplane.coredns.imagePullSecret

Which alternative solutions exist?

Use controlplane.coredns.overwriteManifests and copy default coredns config from coredns.yaml in https://github.com/loft-sh/vcluster/blob/main/chart/templates/coredns-configmap.yaml#L13

Additional context

No response