Add underlying byte data for registers

- change backend logic for register r/w
- remade interface builder pattern for better syntactic sugar
- refined tests

MewTerminal/Commands/OnlineCommands/OnlineCommand.cs

@@ -25,7 +25,7 @@ internal class OnlineCommand : CommandLineExcecuteable {
                 string ip = split[0];
                 int port = int.Parse(split[1]);
 
-                using (var plc = Mewtocol.Ethernet(ip, port)) {
+                using (var plc = Mewtocol.Ethernet(ip, port).Build()) {
 
                     await AfterSetup(plc);