Add a NIO DNS based backend

It would be really great if `swift-async-dns-resolver` could use `DNSClient` (old name "NIO DNS") (https://github.com/orlandos-nl/DNSClient), a platform-independent DNS library built with SwiftNIO as a backend. That way, everything would use the one network stack and we wouldn't need to bind C libraries for the implementation.

Start by reviewing the repository’s current c-ares-based asynchronous DNS implementation and the linked DNSClient project to determine the backend boundary and compatibility requirements. Define what adding this backend means for the public Swift APIs and verify behavior with the repository’s existing tests; the issue does not name specific files or tests.

Written by the indexing model from the issue text.