Support encrypted attachments

Merged tusooa requested to merge tusooa/12-enc-attachment into servant

In this MR we provide a sans-io interface to AES256CTR encryption/decryption (AES256CTRDesc) and SHA256 (SHA256Desc). Also we have an serializable EncryptedFileDesc class to retrieve key, iv and hash from the EncryptedFile json object, and convert such into into a json object.

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