feat: override CoreDNS config

[chris13524]

Proposed Changes

Fixes: #4397 (comment)

Problem:
The coredns-custom configmap does not support overriding an existing server to add custom rules.

Solution:
Import overrides from the coredns-custom ConfigMap.

Types of Changes

New Feature (non-breaking change).

Verification

• deploy this ConfigMap:

  apiVersion: v1
  kind: ConfigMap
  metadata:
    name: coredns-custom
    namespace: kube-system
  data:
    rewrite.override.start: |
      rewrite name example.com traefik.kube-system.svc.cluster.local
  

• verify Traefik's ClusterIP is returned from: kubectl run -it --rm --image=azukiapp/dig dig -- dig example.com

Linked Issues

• [Feature] Customizing CoreDNS ConfigMap with overrides and additional servers #4397
• More extensibility to CoreDNS configmap #462
• [FEATURE] Allow CoreDNS extension with custom configuration k3d-io/k3d#816

User-Facing Change

Allow optional customizations to existing CoreDNS servers via `coredns-custom` ConfigMap

[chris13524]

Closing as per #4397 (comment)

I was able to implement my rewriting needs using:

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns-custom
  namespace: kube-system
data:
  example.server: |
    example.com:53 {
      rewrite name example.com traefik.kube-system.svc.cluster.local
      forward . 127.0.0.1:53
    }