[RFC] CoAP API updates for options

[kb2ma]

Description

Discussion for nanocoap/gcoap API updates to support use of options as defined in #8772. See CoAP API Options Update wiki page for the proposal.

Also see #8920, which triggered the proposal.

[kb2ma]

I just modified the page to require use of coap_opt_finish(), including addition of a parameter to specify that a payload is expected. This addition allows writing the payload marker at this point. Previously, the idea was to maintain the payload marker at all times to avoid use of coap_opt_finish() if no options. I think that approach is too error prone and susceptible to off-by-one.

[kb2ma]

Building up to a WIP PR in my nanocoap/build_request_pkt branch.

[kb2ma]

I just modified the page to remove the 'flags' attribute from coap_pkt_t. Instead, the user requests to sort options via a bitwise parameter in coap_opt_finish(). The addition of the sort function is controlled by a compile time macro.

[kb2ma]

This RFC has served its purpose. #9085 implements the API for nanocoap, and #9156, which partially adapts gcoap for the API, is almost complete. CoAP API Options Update wiki page still is valid and useful.