macOS client does not preserve NextDNS/DoH settings from configuration profile

As [reported in our subreddit](https://www.reddit.com/r/Tailscale/comments/zi6qu0/macos_tailscale_1340_nextdns_issue/), starting with the 1.34 release NextDNS configuration profiles no longer appear to work when using "Use Tailscale DNS Settings" without "Override local DNS".

This likely to a set of DNS-related changes that went into 1.34:
https://github.com/tailscale/tailscale/pull/5731
https://github.com/tailscale/corp/pull/7094
https://github.com/tailscale/corp/pull/7680

NextDNS uses DoH, a configuration profile contains stuff like:

```
DNSSettings

DNSProtocol
HTTPS
ServerURL
https://apple.dns.nextdns.io/NNNNNN/Name

```

We'll need to see if the APIs that we use to read the device DNS settings return DoH configurations too.

(see https://github.com/tailscale/tailscale/issues/4387 for a past macOS/NextDNS quirk, though I don't think it's directly related)