net/nanocoap: add nanocoap_opt2 module

[kb2ma]

Contribution description

#9085 added a struct-based API to nanocoap for adding options to a CoAP message. This PR modularizes that work as nanocoap_opt2, and is the first step to moving all options functions into that context. There are a lot of functions related to options, and moving them to separate implementation files will make them easier to manage. The module in this PR also provides a mechanism to version options functions, so we can deprecate the original function names in favor of a more structured naming scheme.

The idea is to organize options functions like this:

File	Content
opt2_add.c	coap_opt_add_xxx(), struct-based API
opt2_put.c	coap_opt_put_xxx(), buffer-based API
opt2.c	coap_opt_get_xxx(), and generic functionality

Also begins to provide module-level documentation for nanocoap. Describes how to use the struct-based API, and contrasts with the traditional buffer-based API.

Issues/PRs references

Extends #9085

[kb2ma]

@kaspar030, do you have any at least high-level reaction to this PR? I have asked @haukepetersen as part of moving forward on #8920, and I understand @bergzand would like to make a decision and move forward, too.

[kb2ma]

This PR has gotten exactly zero traction from others, so closing for now. I don't want it to hold up other more important PRs like #9156. Perhaps we can revisit at a later date.