feat: ability to pass resolver to lookup MTA-STS policy

[titanism]

Per https://github.com/postalsys/mailauth/blob/29b17af91524fddbd12c3df71ad7d597fd62a996/lib/mta-sts.js#L18 there is support for custom resolver. Perhaps mtaSts options can support a resolver? Or the parent resolver can be passed along that is specified under dnsOptions.resolve?

[andris9]

Passed the resolver option in v1.4.2. Though it does not matter much in the context of MTA-STS as DNS is only a small part of it - most checks are run over HTTPS. DNS query is only used to detect if there is a reason to re-check the policy file, but no actual information is stored in the DNS key.

[titanism]

Thank you @andris9 🙇