Add DNS Configuration Options

[adhilto]

💡 Summary

Add configuration options to allow users to fine-tune how ScubaGear should make the DNS queries needed for the Exchange Online baseline (SPF, DKIM, and DMARC).

Motivation and context

The system default DNS resolver for some users may be unable to resolve the domain names needed. If the users could specify which resolvers to use, they could specify one that will work in their environment.

See #1479 (comment).

Implementation notes

There a lot of different knobs we could add. For example:

• PreferredDnsResolvers: a list of IPs of the DNS resolvers the user wants to use. If not provided, use system default. If provided, use this list. If one resolver fails, try again with the next on list.
• PreferredDoHServers: a list of IPs or domains that should be used for DoH queries.
• PreferDoH: If true, skip the traditional DNS queries. Some users might be using ScubaGear in an environment where the local resolver is unable to resolve the domain (e.g., due to split horizon).
• SkipDoH: If true, skip the DoH queries. Some users might be using ScubaGear where common DoH servers are blocked.

(the above list is far from set in stone but is a good starting place)

Acceptance criteria

DNS configuration options have been added.