Added data_encoder, and data_decoder handlers

[stemann]

Looking into adding something like this to support encryption/decryption...

[MichaelHatherly]

I'd be interested in seeing how this would be used in practise. Just seeing the handlers isn't giving me a clear picture of how they're actually be used to provide encryption support. If you'd prefer to discuss somewhere other than in a public PR then that's fine with me.

[stemann]

The data_encoder and data_decoder handlers can just do something simple, e.g.

function data_encoder(data::Vector{UInt8})::Vector{UInt8}
    encryption_key = parse(UInt8, ENV["SECRET_KEY"])
    return xor.(data, encryption_key)
end

function data_decoder(data::Vector{UInt8})::Vector{UInt8}
    encryption_key = parse(UInt8, ENV["SECRET_KEY"])
    return xor.(data, encryption_key)
end

I.e., doing encryption/decryption using whatever is available in the (default) environment executing the code loading - or using a custom-built Julia with special encryption/decryption-support.