Support Re-Key Operation in AwsCrypto

For scenarios like key-rotation, I think it would be very useful to support a re-keying operation, where we decrypt the data key using one master key, and then re-encrypt with another master key. This also leverages the convenient envelope structure of the encrypted payload. I've noticed this is supported using the KMS Client (re-keying happens on the server side in that case), but feels like it should also exist in the SDK as well!

Thoughts?

Nick

CC: @walliee

首先阅读 AwsCrypto 入口点，并查看 KMS Client 如何在服务器端执行 re-keying。跟踪加密 payload 的 envelope 和数据密钥处理方式，然后确定 SDK 为使用一个主密钥解密并使用另一个主密钥重新加密所需的行为。只有在所提议的操作范围明确且保留加密 payload 的结构时，才算完成。

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