review decrypt API output - add at least encryption context

[mattsb42-aws]

Related to #95, the decrypt API currently only states the decrypted plaintext as an output. However, there are other things that we need to return.

We need to return the keyring trace. #95
We need to return some form of the encryption context (open question: should this be the encryption context that is serialized in the message or the encryption context that the CMM returns?)

Those two are the main things that come to mind, but we should review the API intent to determine if there are additional values that need to be returned.

[WesleyRosenblum]

This seems to conflict with #25, can we close that one?