Allow to send byte[] payload with SAF pub/sub messages.

[berndbr]

Currently the SAF pub/sub messaging only allows to send a payload of datatype string.

Sending binary data packages (e.g. in file transfers, etc.) is not possible and results in converting the binary packages into a JSON string, which will be converted into text using base64 encoding. This is a unnecessary increase of the size of the binary package being send.

To optimize data transfer in certain cases it might therfore be useful to send "byte[]" payloads too.