Add managed DNS record support for API endpoint

[gottwald]

What this PR does / why we need it:
This adds an optional feature to include a managed DNS record that points to the KAS LB and is used as control plane API endpoint in all configs and certs. This makes the IP of the load-balancer interchangeable and makes losing the (ephemeral) LB IP much less fatal to the cluster setup.
It is backwards compatible: meaning if the record is not set, the LB IP is used as it was before.
A transition from IP-based to DNS-based endpoint is not supported in this iteration. A feature like this would also likely need changes in cluster-api itself.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #231

Special notes for your reviewer:
This probably deserves an extension of the e2e test and should be fairly easy to do so, but the e2e environment would need a set up test domain in the e2e DO account to do so. I have no access to that.

Documentation:

Release note:

Add ControlPlaneDNS field in DOClusterSpec which is a managed DNS record that always points to the KAS load-balancer IP and is used as ControlPlaneEndpoint.

[k8s-ci-robot]

Hi @gottwald. Thanks for your PR.

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[timoreimann]

/ok-to-test
/assign @prksu

[prksu]

Just did some testing with this feature and the functionality is work as expected.
I left some comment.

[gottwald]

@prksu PTAL again; please also note that I added a DNS verification thingy which covers an edge case when DNS entry is not propagated, yet. I pushed roughly the same time as you reviewed, so you might not have seen this part, yet.

[prksu]

Maybe we can move this file to another directory like utils in the future. But leave it as is for now

[prksu]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gottwald, prksu

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