Implement Learning and NoLearning trust models

[geonnave]

The draft Implementation Considerations for EDHOC defines three trust models for learning new credentials. Based on a recent conversation with Marco Tiloca, it is likely that the models will be simplified to just two options:

• Learning, where credentials' values or references can be learned during the handshake
• NoLearning, where credentials are pre-stored and only known references or credentials are accepted

This issue aims at experimenting with these models in the implementation.