Support for custom transport layers

[JustTalDevelops]

There is currently a way to listen or dial on a specific network - however, it is fairly limiting with no proper way to register custom transport layers and is non-functional with anything that isn't RakNet.

[Sandertv]

I propose we combine https://pkg.go.dev/image#RegisterFormat and the network parameter in Listen and Dial (https://github.com/Sandertv/gophertunnel/blob/master/minecraft/listener.go#L85). We could have a minecraft.RegisterNetwork function, something like this:

type Network interface {
  ...
}

func RegisterNetwork(name string, n Network) { ... }

to register new networks so that this parameter actually becomes useful.