Helm chart: add an option to override cluster DNS resolver parameters

[atkrv]

Hey there! 👋

I'm using Istio mesh and my cluster domain is different from cluster.local that is set by default with https://github.com/grafana/oncall/tree/dev/helm/oncall

It would be great if chart will support overriding such parameters like dnsService, dnsNamespace and clusterDomain especially. Just like Grafana Mimir guys did in theirs chart - https://github.com/grafana/mimir/blob/481839e1d598a410c3eff15f2a422e7f1405b604/operations/helm/charts/mimir-distributed/values.yaml#L38

Thanks!