dns: add support for Offline Usage with libp2p::SwarmBuilder with_dns

Description

I was running a local build and had with_dns enabled, but I didn't have Wi-Fi in this instance, which it understandably threw me the error.

Error: Custom { kind: Other, error: ResolveError { kind: Proto(ProtoError { kind: Io(Custom { kind: Other, error: "no nameservers found in config" }) }) } }

I'd like to avoid un-commenting it if possible and just have the function with_dns downgrade back to the previous step builder phase in the small case of testing within the builder SawmBuilder with_dns within a local environment without a connection to the internet.

Motivation

• Enable offline peer-to-peer communication for nodes in environments lacking internet access, ensuring that DNS errors don't disrupt functionality.

Current Implementation

There exist not current implementation as I am aware. without commentating it in the instance to which I have no internet

let mut swarm  = libp2p::SwarmBuilder::with_new_identity() 
        .with_tokio()   // tokio runtime
        .with_tcp(      
            tcp::Config::default(), // tcp Config defautlt
            noise::Config::new,     
            yamux::Config::default,
        )? 
        .with_quic()    // quic, and udp on top of ip4/ip6
        // .with_dns()?
        .with_behaviour(|_| stream::Behaviour::new())?  // stream behavioud
        .with_swarm_config(|c| c.with_idle_connection_timeout(Duration::from_secs(10))) // duration of stream lasts 10 seconds until given up
        .build();

Are you planning to do it yourself in a pull request?

Yes

Start by reading the libp2p::SwarmBuilder::with_dns entry point and the provided builder example, focusing on how DNS setup currently handles a missing nameserver. Verify the intended behavior in an offline environment: enabling with_dns should not prevent the builder from reaching the behaviour and build stages when DNS is unavailable.