refactor(dns): unify symbol naming

Renamed the following
- `dns::GenDnsConfig`  ->  `dns::Config`
- `dns::DnsConfig` -> `dns::async_std::Config`
- `dns::TokioDnsConfig` -> `dns::tokio::Config`

If async-std feature is enable, use `dns::async_std::Config`. When using tokio, import `dns::tokio::Config` . There is no need to use `dns::Config` directly.

Resolves #4486.
Related: #2217.

Pull-Request: #4505.

examples/dcutr/src/main.rs

@@ -33,9 +33,7 @@ use libp2p::{
         transport::Transport,
         upgrade,
     },
-    dcutr,
-    dns::DnsConfig,
-    identify, identity, noise, ping, quic, relay,
+    dcutr, dns, identify, identity, noise, ping, quic, relay,
     swarm::{NetworkBehaviour, SwarmBuilder, SwarmEvent},
     tcp, yamux, PeerId,
 };
@@ -102,7 +100,7 @@ fn main() -> Result<(), Box<dyn Error>> {
                 &local_key,
             )));
 
-        block_on(DnsConfig::system(relay_tcp_quic_transport))
+        block_on(dns::async_std::Transport::system(relay_tcp_quic_transport))
             .unwrap()
             .map(|either_output, _| match either_output {
                 Either::Left((peer_id, muxer)) => (peer_id, StreamMuxerBox::new(muxer)),