Add options to directly contact hosts by name or ip + disable broadcast

[VolodiaPG]

Following options are added

• extraHosts enables the specification of hostnames/ips to specifically contact
• disableBroadcast would avoid to use the broadcast addresses from the interfaces by default

Those options are added using two env variables:

• comma-separated PEERIX_EXTRA_HOSTS
• boolean PEERIX_DISABLE_BROADCAST (true or false in string)

Also puts a small example in the README.MD.

Use case:

1. Use a VPN such as tailscale, aka a mesh vpn to connect multiple computers together, independantly of the network
2. Broadcast interfaces are not a feature of any VPN
3. Peerix can still be used accross networks thanks to a module defined such as:

    _: {
      services.peerix = {
        enable = true;
        openFirewall = true; # UDP/12304
   
        privateKeyFile = ../secrets/peerix-private;
        publicKeyFile = ../secrets/peerix-public;
   
        user = "peerix";
        group = "peerix";
   
        disableBroadcast = true;
        extraHosts = [ "my-nixos-computer-hostname" "42.42.42.1" ];
      };
      users.users.peerix = {
        isSystemUser = true;
        group = "peerix";
      };
      users.groups.peerix = { };
    }