Extend grasp registration

[mcr]

Added IANA Considerations for GRASP Discovery extension for RJP.

[mcr]

So, this moves to discovery document, and this PR should be dropped?

[EskoDijk]

I think the plan was to all discovery work to the discovery-draft, except for one (single) "default method" which is necessary to make the solution work. This default method will be an existing discovery method without defining how to support discovery of protocol variations.

In other words: the Pledge discovers the Constrained Join Proxy (CJP), and assumes it will work for its protocol variant. And the CJP discovers the Registrar (if stateful CJP) or the Registrar's JPY protocol front-end (if stateless CJP) and assumes it will work for anything that it got from a Pledge.

In the calls we proposed to make CoAP discovery this default method. In that case and if we go along with this solution, GRASP would be fully removed and left as "future work". Also we don't detail any of the much better discovery methods for CJP to use such as Service Registration Protocol based discovery (using DNS-SD), or CoRE Resource Directory.