Feature request: add option to immediately validate ciphertext by decrypting it

[farleyb-amazon]

When encrypting data to create an encrypted message, some customers may want to immediately attempt a decryption of the data before returning it in order to confirm that the ciphertext is valid and can be decrypted. We could offer this as a built-in option of the ESDK; during encrypt, we internally decrypt the encrypted message before returning it.

We'll have to think a bit about the API here to come up with something that works for all supported languages. See one possible approach here: aws/aws-encryption-sdk-java#402

See also: aws/aws-encryption-sdk-java#279