poc binary type

Binary type with data & size. I reckon we may already need this with wifi passphrase which can be a 64c hexadecimal PSK or a string 63c or less.

Checking null termination in this case would work because the hexadecimal value cannot have '\0' but in the future we might encounter pure binary types for which length needs to be sent.

We can of course add a new field call something_size in there, but this feels more elegant.