nanocoap: Add handler for resource-based subtrees

[bergzand]

Contribution description

This adds a coap_handler_t function that can be used to parse new
subtrees. The subtree information is included in the context pointer of
the call and must be of type coap_resource_subtree_t. This object then
contains the pointer and length of a different coap_resource_t instance.

Testing procedure

The coap handlers in examples/suit-update should still work as before

Issues/PRs references

Follow-up with more cleanups to #13687

[kb2ma]

This PR looks like a step toward a concept like gcoap_listener_t. Eventually it will make sense to add more metadata to a coap_subtree_handler_t, and to use it in place of the externs for coap_resources and coap_resources_numof.

I created #11488 to move this sort of application level functionality to nanocoap sock to avoid duplication and confusion with gcoap. The contents of the current PR are similar to #11436 from last year that was closed partly due to these concerns.

As this coap_subtree_handler_t matures and we find commonality between nanocoap sock and gcoap, we can move the common bits into nanocoap itself. Maybe we can at least move coap_handle_req() and its components to nanocoap sock for now.

[benpicco]

Only moves code from SUIT into the common domain.

[kb2ma]

I would appreciate a response to my concerns before merging.

no longer a stake holder

[miri64]

@bergzand can you add the response requested by Ken?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.

[benpicco]

This is still a valid cleanup.
Just because there was a larger cleanup underway (that is now stalled) should not stop this from being merged.

Please rebase.

[OlegHahm]

@bergzand ping