Add a "retrieve all" mode for wgsd-client

[PoneyClairDeLune]

Adding a "retrieve all" mode, and trigger it via -mode all flag. This will allow the registry to act as a peer distributor, greatly reducing the effort for building a mesh-based network.

This mode will make wgsd an all-in-one solution to replace n2n fully, and replace Tailscale to a certain extent.