Add option to prefer IPv6 resolving (#6494)

This PR adds an option to prefer IPv6 resolving before IPv4.
On IPv6 only systems this could be very useful, but will not solve IPv4 only domains of course.
For that you need a DNS64 + NAT64 solution

Fixes #6301

Signed-off-by: BlackDex <black.dex@gmail.com>

src/http_client.rs

@@ -185,7 +185,10 @@ impl CustomDnsResolver {
 
     fn new() -> Arc<Self> {
         match TokioResolver::builder(TokioConnectionProvider::default()) {
-            Ok(builder) => {
+            Ok(mut builder) => {
+                if CONFIG.dns_prefer_ipv6() {
+                    builder.options_mut().ip_strategy = hickory_resolver::config::LookupIpStrategy::Ipv6thenIpv4;
+                }
                 let resolver = builder.build();
                 Arc::new(Self::Hickory(Arc::new(resolver)))
             }