Add: Implement methods to find users by UserId and username

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

@@ -1,6 +1,8 @@
 package ch.unibas.dmi.dbis.cs108.casono.server.domain.user;
 
 import ch.unibas.dmi.dbis.cs108.casono.server.network.sessions.SessionId;
+
+import java.lang.classfile.attribute.UnknownAttribute;
 import java.util.Collection;
 import java.util.Optional;
 import java.util.concurrent.ConcurrentHashMap;
@@ -95,6 +97,14 @@ public class UserRegistry {
         return Optional.ofNullable(bySessionId.get(sessionId));
     }
 
+    public Optional<User> findByUserId(UserId userId) {
+        return Optional.ofNullable(byId.get(userId));
+    }
+
+    public Optional<User> findByUsername(String username) {
+        return Optional.ofNullable(byName.get(username));
+    }
+
     /**
      * Returns all currently registered users.
      *