Add: ListUsersHandler to handle user listing command

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/list_users/ListUsersHandler.java

@@ -0,0 +1,22 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.list_users;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.User;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+import java.util.Collection;
+
+public class ListUsersHandler extends CommandHandler<ListUsersRequest> {
+    private final UserRegistry userRegistry;
+
+    public ListUsersHandler(ResponseDispatcher responseDispatcher, UserRegistry userRegistry) {
+        super(responseDispatcher);
+        this.userRegistry = userRegistry;
+    }
+
+    @Override
+    public void execute(ListUsersRequest request) {
+        Collection<User> users = userRegistry.getAllUsers();
+        responseDispatcher.dispatch(new ListUsersResponse(request.getContext(), users));
+    }
+}