[Feature Request] `UdpRequestResolver` without any fallback

[GF-Huang]

Sometime we don't need auto fallback in UDP query, just simple respond the Truncated response. Now, if we don't set the fallback, it will always throw an exception by NullRequestResolver.

dns/DNS/Client/RequestResolver/UdpRequestResolver.cs

Lines 42 to 44 in 8094ac5

	if (response.Truncated) {
	return await fallback.Resolve(request, cancellationToken).ConfigureAwait(false);
	}

dns/DNS/Client/RequestResolver/NullRequestResolver.cs

Lines 6 to 10 in 8094ac5

	public class NullRequestResolver : IRequestResolver {
	public Task<IResponse> Resolve(IRequest request, CancellationToken cancellationToken = default(CancellationToken)) {
	throw new ResponseException("Request failed");
	}
	}