Validation of YAML root structure for WIP SplitDNS YAML config

[JosiahWI]

This contains just the file loading and root validation part of a YAML loader for SplitDNS. I foresee that the complete feature will be a large change, and I would like feedback now while the changes are small to help guide my design and to increase the effectiveness of the code review.

I am marking this as a draft PR because all these changes are 'dead code' and are not useful yet by themselves.

[bneradt]

[approve ci rat]

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. Marking it stale to flag it for further consideration by the community.