Implemented Cache for decrypted private key and handled refresh token

src/main/java/com/skycrate/backend/skycrateBackend/services/KeyCacheService.java

@@ -0,0 +1,28 @@
+package com.skycrate.backend.skycrateBackend.services;
+
+import org.springframework.stereotype.Service;
+
+import java.util.concurrent.ConcurrentHashMap;
+
+@Service
+public class KeyCacheService {
+
+    private final ConcurrentHashMap<Long, String> keyCache = new ConcurrentHashMap<>();
+
+    public void cacheKey(Long userId, String decryptedKey) {
+        keyCache.put(userId, decryptedKey);
+    }
+
+    public String getKey(Long userId) {
+        return keyCache.get(userId);
+    }
+
+    public void clearKey(Long userId) {
+        keyCache.remove(userId);
+    }
+
+    public void clearAllKeys() {
+        keyCache.clear();
+    }
+}
+