Expose MasterKey used for Decryption as part of Decrypt result

[SalusaSecondus]

When decrypting data with the SDK, we must expose which master key was used to decrypt the data so that the caller can use it in program logic.

[farleyb-amazon]

With recent changes to our keyring spec, as well as changes to our best practices (specifically the guidance around using strict providers and only specifying keys you're willing to decrypt with), this is no longer relevant. Resolving.