Put coredns configuration in a configmap

[michaelbeaumont]

Description

This would give users

1. visibility
2. configurability

into how we configure coredns to handle DNS requests in the mesh

[lahabana]

Only question is universal. Having it embedded is nice for universal

[jakubdyszkiewicz]

Triage: it's still useful to have this embedded in kuma-dp run

[lahabana]

xref: #8269

[jijiechen]

Replacing #8633

More background:

Description

People want to customize the DNS template used by the kuma data plane to address following issues:

• Integrate with their existing DNS servers
• Resolve certain network issues introduced by the DNS proxying model of kuma dp, e.g. it's found by some Kuma clients that the DNS in kuma data plane is not compatible with certain alpine images (github.com not reachable from alpine image alpinelinux/docker-alpine#155 (comment))

It's good to make our builtin Corefile template customizable.

It's best to support for both k8s and universal data planes.

Not in scope:

configuring command line of CoreDNS at data planes parameters from the control plane

[jijiechen]

The PR has now been merged and we'll see if we need to backport to older versions.