Add support for encryption of binary data between SOH and ETX

[Scottapotamas]

Allow the developer to provide a key or value which is used for some light crypto (optionally), to provide encrypted transport.

This would involve a minor refactor of message generation and ingest functions to support a compile time flag to enable encryption of messages.

[Scottapotamas]

This has been discussed a bit over the last week or so.

We don't really want to have this be a library feature, but there are items which can make it easier to swap out the protocol. See #77 etc.

The developer really should just provide the input/output scratchpad memory and do their own encoding as required.