node-local-dns should support graceful shutdown with the ready plugin

[jsws]

We are using node-local-dns with a CiliumLocal Redirect Policy as described here so we need node-local-dns to be able to become Not Ready once it receives a SIGTERM so time is given for the Endpoint to stop receiving traffic and for requests to be directed to kube-dns instead, before the pod is fully terminated. This will allow graceful rollouts of the node-local-dns daemonset.

In #485 it was requested the ready plugin be added to node-local-dns but it was closed on the basis it wouldn't serve any purpose as it would behave the same way as the health plugin. However I believe that's not the case and the ready plugin would allow node-local-dns to gracefully shutdown, since once it receives a SIGTERM the /ready endpoint would start returning 503 which can be used in a Readiness Probe on the pod to determine the pod is shutting down and give time for the Endpoint to be removed when used with the health plugins lameduck setting.
So I am again requesting the ready plugin be added to the plugins list here.

[LeoQuote]

Hi, I’m currently exploring this option( cilium + nodelocal dns), but I see no reason to use image provided by this project instead of using coredns directly, did you try to use coredns directly? Is there any hidden disadvantages?

I tried to use coredns instead and even drafted a PR for this. Hope that would help you coredns/helm#173

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[kevin2000141]

Hi, I'm creating this PR to solve the readiness issue.

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