Offline key caching

A customer goes out to a field office, that is off the grid. They want to be able to encrypt things with a key, but they can't talk to KMS. The idea would be they could fill up some cache of keys prior to going offline, then when they need to encrypt data, they can use those keys.

未指定任何檔案、測試或入口點。首先釐清離線金鑰快取的需求，包括快取行為和金鑰管理限制，然後找出相關的加密和 KMS 整合區域。完成的標準是，使用者可以在中斷連線前預先快取金鑰，並在離線時加密資料。

由索引模型根據 Issue 內容生成。