Add functions, roles, and object models to draft (#297)

* Add functions, roles, and object models to draft

This updates IETF-RFC.md with a significantly expanded conceptual
framework for OCM. It introduces formal definitions of OCM functions,
clarifies the roles of Sending and Receiving Servers, and adds detailed
object models for Address Books, Contacts, Invites, Providers, Shares,
Users, and Resources. The change reorganizes and expands terminology,
adds diagrams to illustrate relationships, and alphabetizes and
harmonizes defined terms. Legacy duplicated sections are removed or
merged into the new structure.

Signed-off-by: Micke Nordin <[email protected]>

* Address feedback from @KrausMatthias

* Remove sequence diagram better supplied elsewhere

* Move object model to appendix D

* Address feedback from Giuseppe

Co-authored-by: Giuseppe Lo Presti <[email protected]>

* Add back functions and roles

---------

Signed-off-by: Micke Nordin <[email protected]>
Co-authored-by: Giuseppe Lo Presti <[email protected]>

Author: mickenordin