fix(examples): call Swarm::add_external_address in dcutr and relay

> Observed addresses (aka. external address candidates) of the local node, reported by a remote node
> via `libp2p-identify`, are no longer automatically considered confirmed external addresses, in
> other words they are no longer trusted by default. Instead users need to confirm the reported
> observed address either manually, or by using `libp2p-autonat`. In trusted environments users can
> simply extract observed addresses from a `libp2p-identify::Event::Received { info:
> libp2p_identify::Info { observed_addr }}` and confirm them via `Swarm::add_external_address`.

Follow-up to https://github.com/libp2p/rust-libp2p/pull/3954.

Pull-Request: #4052.

examples/dcutr/src/main.rs

@@ -214,6 +214,7 @@ fn main() -> Result<(), Box<dyn Error>> {
                     ..
                 })) => {
                     info!("Relay told us our public address: {:?}", observed_addr);
+                    swarm.add_external_address(observed_addr);
                     learned_observed_addr = true;
                 }
                 event => panic!("{event:?}"),