Refactor: Change CommandHandler implementations to extend the abstract class

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/check_nick/CheckUsernameHandler.java

@@ -7,7 +7,7 @@ import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatch
 import java.util.Optional;
 
 /** Handles {@link CheckUsernameRequest}s to check whether a username is available. */
-public class CheckUsernameHandler implements CommandHandler<CheckUsernameRequest> {
+public class CheckUsernameHandler extends CommandHandler<CheckUsernameRequest> {
     private final ResponseDispatcher responseDispatcher;
     private final UserRegistry userRegistry;
 

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/login/LoginHandler.java

@@ -13,7 +13,7 @@ import java.util.Optional;
  * Handles {@link LoginRequest}s to create a user for a session, if the session has not assigned one
  * already
  */
-public class LoginHandler implements CommandHandler<LoginRequest> {
+public class LoginHandler extends CommandHandler<LoginRequest> {
     private final ResponseDispatcher responseDispatcher;
     private final UserRegistry userRegistry;
     private final UserFactory userFactory;

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/logout/LogoutHandler.java

@@ -7,7 +7,7 @@ import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.OkRespon
 import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
 
 /** Handles {@link LogoutRequest} to logout connected user. */
-public class LogoutHandler implements CommandHandler<LogoutRequest> {
+public class LogoutHandler extends CommandHandler<LogoutRequest> {
     private final ResponseDispatcher responseDispatcher;
     private final UserRegistry userRegistry;
 

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/ping/PingHandler.java

@@ -5,7 +5,7 @@ import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.OkRespon
 import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
 
 /** Handler for {@link PingRequest}. */
-public class PingHandler implements CommandHandler<PingRequest> {
+public class PingHandler extends CommandHandler<PingRequest> {
     private final ResponseDispatcher responseDispatcher;
 
     /**