Try #2: Support encrypted DNS txt records

[HansK-p]

Once again creating a PR for this change. I managed to foobar my previous PR earlier today as my git experience is extremely limited.

We are running on a fork of External-DNS where txt records are encrypted. The encryption is an internal security requirement as we are not allowed to leak information about internal infrastructure through publicly available txt records.

It is a pain to keep a forked version of External-DNS up to date and it would be really nice if this PR could be considered (or at least looked at). The previous PR was created on December 6th 2019 and was absolutely, totally and completely ignored by the project maintainers for more than 4 months.

We are looking for other solutions, but this forked version of External-DNS is currently our best option. So we really hope txt record encryption will be included as a security feature in External-DNS. It doesn't mater a lot for us this code is used as a base for implementation - as long as it is implemented.

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[HansK-p]

/assign @njuettner

[k8s-ci-robot]

@HansK-p: PR needs rebase.

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[HansK-p]

I'll rebase when asked to do so by a human :) or when we upgrade our own installation. So the PR will be rebased when needed.......

[Hugome]

+1 for this feature. I also need sometime this kind of ""overkill"" security feature for sensitive project. And for the moment I cannot use external-dns in these projects :(.
(And +1 @HansK-p nice clean code & tests :) maybe to be picky this miss a little of docs/example ?)

[seanmalloy]

/kind feature

[HansK-p]

And please ping me if I should solve conflicts. It's a small pain to solve those conflicts, so I will only do it when we decide to upgrade our modified internal external-dns installation or when asked by a human.

[seanmalloy]

And please ping me if I should solve conflicts. It's a small pain to solve those conflicts, so I will only do it when we decide to upgrade our modified internal external-dns installation or when asked by a human.

@HansK-p thanks for the heads up. I can't get to this PR right now. We will let you know when we are ready to review it. Thanks!

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