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.
* Fixed an indexOutOfRange warning when creating coap requests
because the first addOption() call was calling
m_options.insert(1) on an empty list.
* Old code was appending multiple UDP datagrams to a single big Coap
message, however, Coap is specified to only send s single datagram
per message. The datagram length specifies the payload size.
* some boolean member variables weren't initialized which resulted in
occational wrong flags.
* Parsing had issues with determining the option length in some occations
and also would crash when receiving coap messages without any options
or payload. To get rid of the complex and erraneous index calculations,
the entire package parsing is now using a DataStream.
(This makes it work with Shelly devices)
