feat(pdns): add validation for MX and SRV records

[julillae]

Description
Adds a check to validate the format of MX and SRV records. In addition, implements AdjustEndpoints to filter incorrectly formatted records which we know will fail to be created by PowerDNS. This change improves error handling for MX and SRV records and provides a base to validate other record types in the future.

Fixes #ISSUE

Checklist

• Unit tests updated
• End user documentation updated

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[k8s-ci-robot]

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[mloiseleur]

Thanks for this PR, which is tested.
I'm wondering: the format check based on RFC may be used by other providers.
Wdyt of moving the checkEndpoint part in endpoint/endpoint.go ?

[mloiseleur]

/retitle feat(pdns): add validation for MX and SRV records

[julillae]

@mloiseleur That is a great point! I moved the checkEndpoint function to endpoint/endpoint.go.