CoIoT is a shelly specific extensions to CoAP:
It adds a new Request code which is not part of the CoAP spec
as well as using CoAP multicast (which is part of CoAP).
This commit
* renames "statusCode" to "reqRspCode" which describes
the actual field more precisely as it is in fact a Request or Response
code, not a status code.
* Allows joining multicast groups, by default using the CoAP specified
multicast address.
* Allows setting custom Request codes in requests, as well as
processing the PDUs Request/response code by the client.
