feat(coredns): Forward extra domains to coredns kubernetes plugin

[mvandergiesen]

What type of PR is this?
/kind feature

What this PR does / why we need it:
For our environment we would like to have a more granular method to specify the domains passed to the coredns kubernetes plugin. This pull-request adds an extra optional variable that will help users who want to fine-tune the configuration for a specific environment.

Does this PR introduce a user-facing change?:

Add the possibility to specify extra domains for the coredns kubernets plugin.

[k8s-ci-robot]

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[floryut]

@mvandergiesen Could you rebase as #9640 is now merged (Fixing the cri-o CI) ?

[oomichi]

The change itself seems good for me.

/approve

[floryut]

/lgtm
/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, mvandergiesen, oomichi

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