Add mysql_extend external plugin

[snail2sky]

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Note:

• All text listed in /plugins is a copied from https://github.com/coredns/coredns/tree/master/plugin.

• The release notes are copied from https://github.com/coredns/coredns/tree/master/notes.

Any pull request that updates either of those should be directed to the CoreDNS repository: https://github.com/coredns/coredns .

[snail2sky]

Please add this as an external plugin per the following instructions: https://coredns.io/2017/07/23/add-external-plugins/

Have been adjusted