Add: UserId to uniquely identify user

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/user/UserId.java

@@ -0,0 +1,31 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.domain.user;
+
+import java.util.UUID;
+
+/** Represents a unique identifier for a user. */
+public class UserId {
+    private final UUID value;
+
+    /** Creates a new UserId with a randomly generated UUID. */
+    public UserId() {
+        this.value = UUID.randomUUID();
+    }
+
+    /**
+     * Creates a new UserId with the specified UUID.
+     *
+     * @param value the UUID to use for this UserId
+     */
+    public UserId(UUID value) {
+        this.value = value;
+    }
+
+    /**
+     * Returns the UUID value of this UserId.
+     *
+     * @return the UUID value
+     */
+    public UUID value() {
+        return value;
+    }
+}