Add: Change CommandHandler to abstract class and add support for pre-execution checks

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/network/command/execution/CommandHandler.java

@@ -1,7 +1,21 @@
 package ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution;
 
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.checks.HandlerCheck;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
 
-public interface CommandHandler<T extends Request> {
+public abstract class CommandHandler<T extends Request> {
-    void execute(T request);
+    private final List<HandlerCheck> checks = new ArrayList<>();
+
+    protected final void addCheck(HandlerCheck check) {
+        checks.add(check);
+    }
+
+    public final List<HandlerCheck> getChecks() {
+        return Collections.unmodifiableList(checks);
+    }
+
+    public void execute(T request) {}
 }