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.

未指定任何文件、测试或入口点。首先明确离线密钥缓存的要求，包括缓存行为和密钥管理约束，然后确定相关的加密和 KMS 集成区域。完成的标准是，用户可以在断开连接前预缓存密钥，并在离线时加密数据。

由索引模型根据 Issue 内容生成。