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

Start by reading the AwsCrypto entry point and reviewing how the KMS Client performs re-keying on the server side. Trace the encrypted payload's envelope and data-key handling, then determine the SDK behavior needed to decrypt with one master key and re-encrypt with another. Done means the proposed operation is clearly scoped and preserves the encrypted payload structure.

Written by the indexing model from the issue text.