net/unicoap: basic CoAP group communication support

[elenaf9]

Contribution description

WIP group communication support for unicoap.
Group communication is based on IP multicast, with non-confirmable messages. Most of it works already out-of-the-box using the gnrc IP multicast support. Two main changes required:

• If a request was multicast, we can expect multiple replies. So only clear the memo after a timeout or if the user manually aborted the request.
• Add the timeout for multicast requests, until which replies are acceptable.

Testing procedure

Not tested yet because there seems to be a bug in the client implementation.

Issues/PRs references

Small addition to #22266.

Tracking issue: #21389

Declaration of AI-Tools / LLMs usage:

AI-Tools / LLMs that were used are:

• none

[riot-ci]

Murdock results

❌ FAILED

993cf29 unicoap: basic group communication support

Success	Failures	Total	Runtime
2893	0	10410	03m:39s

Artifacts

[carl-tud]

Couple of small things; would it be possible to cancel a multicast request using unicoap_cancel?

[mguetschow]

I assume this is an implementation of https://datatracker.ietf.org/doc/html/rfc7390? I was yesterday pointed to https://datatracker.ietf.org/doc/draft-ietf-core-groupcomm-bis/ which obsoletes RFC7390 with many more features. Might be worth to have a look. It is about to be published as RFC: https://queue.rfc-editor.org/#draft-ietf-core-groupcomm-bis