All: Investigate better way of abstracting envelope encryption implementation

[jpaskhay]

Currently EnvelopeEncryptionJsonImpl contains the core envelope encryption algorithm. We also have a wrapper EnvelopeEncryptionBytesImpl that sits as a wrapper around that class. This was done as a quick fix for allowing a different serialization mechanism for the DRR. Investigate if we can abstract this a bit more to avoid having a hard-coded dependency. May be addressed as part of #14.