Use DCUtR to synchronize hole punching

[p-shahi]

• Spec: https://github.com/libp2p/specs/blob/master/relay/DCUtR.md

[BigLep]

@p-shahi : I assume this is needed for browsers being able to dial or be dialable by server nodes with WebRTC. Is that right?

[SgtPooki]

related: ipfs/helia#180

[aschmahmann]

Related to the linked issue(s) above. If implementing synchronized hole punching via DCUtR is going to take a while, it's probably worth doing the minimal amount of DCUtR that enables dialbacks to nodes that are publicly reachable.

While it's only a subset, it's likely an easier subset and unlocks js-libp2p nodes (including those in browsers) being reachable by any libp2p node which is itself publicly reachable (on a shared transport). This enables a path forward for users to build systems where their browser nodes are reachable from non-browser nodes.