cmd/{k8s-nameserver,k8s-operator},k8s-operator: add a kube nameserver, make operator deploy it

This is https://github.com/tailscale/tailscale/pull/11017 that was already once merged, but I reverted it just before 1.64 release in https://github.com/tailscale/tailscale/pull/11669 as we did not have the bandwidth to review the second part (https://github.com/tailscale/tailscale/pull/11019) and I did not want to release half of a feature. Below is the original PR description, the contents haven't changed:

This PR is the first part of the work towards enabling support for MagicDNS name resolution from within Kubernetes cluster. Second part that updates the nameserver config with 'DNS records' is in https://github.com/tailscale/tailscale/pull/11019. This PR:

implements a simple nameserver, that reads hosts records from a file and responds to A record DNS queries over UDP and TCP
adds a DNSConfig custom resource that can be used to tell the operator to deploy the nameserver
Adds a new control loop that deploys/syncs nameserver resources in response to users creating DNSConfigs
adds some infra for nameserver (make target etc) and CRD generation

Operator advertizes the nameserver's Service IP address on the DNSConfig status. Users can read it from there to update clusterDNS. Example flow:

Apply a DNSConfig:

apiVersion: tailscale.com/v1alpha1
kind: DNSConfig 
metadata:
name: ts-dns
spec:
nameserver:
image:
  repo: gcr.io/csi-test-290908/nameserver // official images are not published yet- this is from a registry of mine
  tag: v0.0.64dns

Wait for the DNSConfig's status to be updated:

$ kubectl get dnsconfig ts-dns
NAME     NAMESERVERIP
ts-dns   10.44.11.171

Update cluster DNS to add a stub nameserver for ts.net (the following example is for kube-dns, Corefile has different syntax):

apiVersion: v1
data:
  stubDomains: |
    {
      "ts.net": ["10.44.11.171"]
    }
kind: ConfigMap
metadata:
  labels:
    addonmanager.kubernetes.io/mode: EnsureExists
  name: kube-dns
  namespace: kube-system

tailscale / tailscale

cmd/{k8s-nameserver,k8s-operator},k8s-operator: add a kube nameserver, make operator deploy it #11919