Change coredns extra_plugins to array type

[infbase]

• Specify extra_plugins of coredns as array instead of plain string

• Adjust schema

• Enables more intuitive and convenient plugin specification, e.g

coredns:
    extra_plugins:
      - log
      - prometheus

• Partially fixes #1267

..

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[ytsarev]

@kuritka @k0da , guys, I need your review here. do you use original string representation to construct the Corefile config?

[k0da]

LGTM