net: coap: Allow encoding packets using data from constant buffers

Token and payload are appended from data buffers to a CoAP packet
being encoded. Keyword const was missing for parameters in functions
appending these parts to a packet.

Now token and paylod can be copied to CoAP packet from constant
buffers, that can be stored in ROM.

Signed-off-by: Hubert Miś <hubert.mis@nordicsemi.no>