Add section Optimization Strategies

[geonnave]

Summary of the changes:

new Section 5: Optimization Strategies, where broadcast and advertisement -based solutions are presented:

• 5.1 U anycasts message_1 (previously approach A4)
  • explains the approach and mention associated security/congestion considerations.
• 5.2 V advertises support for ELA:
  • describe the idea of adversiting ELA (sharing a V_INFO struct, whose definition depends on network/application)
  • propose ELA_ID and DOMAIN_ID as guideline for implementers.

new Appendix A: use with EDHOC reverse flow (Goran's PR #36)

new Appendix C: advertisement strategies
(sampled from what we have discussed at length already)

• C.1. V_INFO in network beacons (previously approach A1)
• C.2. V_INFO in EAD_1 (previously approach A2)
• C.3. V_INFO in a CoAP Multicast Packet (previously approach A3)

[geonnave]

May still do some changes, but I think it can already receive reviews.

[geonnave]

Merging -- changes were discussed in meeting with co-authors @gselander and @malishav on Oct 16.