sys/net/app/cord: Support the addition of extra registration arguments

[chrysn]

Contribution description

Registations at a Resouirce Directory can have extra arguments; this is useful for example to ask an RD to reverse-proxy you.

Testing procedure

With a board that gets a route to the Internet, add the following config to one of examples/cord_ep or examples/cord_epsim, and run it.

+CFLAGS += '-DCONFIG_CORD_EXTRAARGS="proxy=on","et=tag:riot-os.org,2020:board"'
+CFLAGS += '-DCONFIG_CORD_EP="myhost"'

The epsim version also needs

-RD_ADDR ?= \"[affe::1]\"
+RD_ADDR ?= \"[2a01:4f8:190:3064::5]\"

whereas the non-simple version needs the command

> cord_ep register [2a01:4f8:190:3064::5]

executed.

Then, you should see your device registering with a synthetic proxy URI (courtesy of the proxy=on argument) in the public RD indicated by the addresses:

$ ./aiocoap-client coap://rd.coap.amsuess.com/endpoint-lookup/ --observe
</reg/2/>; ep="myhost"; et="tag:riot-os.org,2020:board"; base="coap://myhost.proxy.rd.coap.amsuess.com"; rt="core.rd-ep"

(The endpoint name is added manually because this particular RD refuses proxying for devices with non-lowercase endpoint IDs to avoid the hassle of disambiguating the case insensitive host names).

Issues/PRs references

This has all the commits of #16112 in it (as the simple version would otherwise register to an outdated address); the latest commit is what counts.

[edit: all dependencies merged, only single commit left over]

[chrysn]

Rebased onto master to be testable more easily. Own tests still pass, please review, this is really small.

[benpicco]

yea sure, why not

[chrysn]

Thanks :-)