Change how the validation label is constructed

[aaomidi]

Addresses #13

Trying to get closer to the generic dns validations method document: https://datatracker.ietf.org/doc/draft-ietf-dnsop-domain-verification-techniques/

[github-actions[bot]]

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[daknob]

Seems okay so far, pending the changes from the dnsop draft. We'd like to cite that somewhere.