Add: TransportLayer interface to abstract direct socket access

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/network/TransportLayer.java

@@ -0,0 +1,9 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.network;
+
+import java.io.IOException;
+
+public interface TransportLayer {
+    String read() throws IOException;
+    void write(String data) throws IOException;
+    void close() throws IOException;
+}