Add: SessionManager to store all active sessions

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/network/SessionManager.java

@@ -0,0 +1,28 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.network;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class SessionManager {
+    private Map<SessionId, Session> sessions;
+
+    public SessionManager() {
+        this.sessions = new ConcurrentHashMap<>();
+    }
+
+    public void addSession(Session session) {
+        sessions.put(session.getId(), session);
+    }
+
+    public Session removeSession(SessionId id) {
+        return sessions.remove(id);
+    }
+
+    public Session removeSession(Session session) {
+        return sessions.remove(session.getId());
+    }
+
+    public Session getSessionById(SessionId id) {
+        return sessions.get(id);
+    }
+}