Feature: IPv6 support

[yorickdowne]

Is your feature request related to a problem? Please describe.

With CGNAT becoming more prevalent, peering is hard for users when IPv4 incoming connections are not available.

Describe the solution you'd like

Support IPv6 for P2P. This could be done by doing dual-stack by default, and as well by allowing one v4 and one v6 --listenAddress. Lighthouse for example does --listen-address 0.0.0.0 --listen-address :: for dual-stack, and specifying only either gets you a v4 or v6 solo stack.

Additional context

P2P support is the driver, here. Metrics, REST, key manager, all can stay v4, at least initially. If it's trivial to make them dual-stack as well, by all means do, but for the bulk of users where this is an issue - home users with CGNAT - they'll have v4 available and they won't need to reach REST via v6 from "the Internet"