Motivation:
`DnsServerAddressStreamProviders` tries to load `MacOSDnsServerAddressStreamProvider`
on macOS. However, it doesn't warn users when `MacOSDnsServerAddressStreamProvider`
is not awailable, which may cause incorrect results for DNS resolutions.
Modifications:
- Log at warn level if `MacOSDnsServerAddressStreamProvider` is not found on macOS;
- Log at debug level when `MacOSDnsServerAddressStreamProvider` is loaded and available;
Result:
macOS users are notified when `MacOSDnsServerAddressStreamProvider` is not available.
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