S3 Encryption Client payload interoperability

### Problem:

My organisation currently uses S3 Encryption Client for client-side encryption of our S3 objects.
We need to support multiple blob storages (such as GCS) with client-side encryption.

S3 Encryption Client is only compatible with S3. We're looking to have a solution that will be multi-storage.

### Solution:

One option we are evaluating is relying on S3 Encryption Client internals such as CipherSubscriber/CipherProvider (a couple more) and building a thin agnostic wrapper around it such that we can continue to use it for all our existing S3 objects.

However being internal API we are hesistant. Would it be possible to promote some of the internal API to public?

We have also considered AWS Encryption SDK, but it isn't interoperable with the payloads due to its framed format. We'd also appreciate any other suggestions! Re-encryption to a new format would also be very expensive.

首先检查 issue 中提到的 S3 Encryption Client 内部实现，尤其是 CipherSubscriber 和 CipherProvider，并将其 payload 格式与 AWS Encryption SDK 的分帧格式进行比较。未确定任何文件或测试；要视为完成，需要达成一致的公开、与存储无关的 API，或采用其他能够保留现有 S3 加密 payload 的互操作方案。

由索引模型根据 Issue 内容生成。