Added initial support for coap options Observe, Conditional Observe and High Level States.


As described in
https://datatracker.ietf.org/doc/draft-li-core-conditional-observe/ (Version 04)
https://datatracker.ietf.org/doc/draft-ietf-core-observe/ (Version 06)
https://datatracker.ietf.org/doc/draft-mietz-core-coap-state-option/ (Version 00)
 
Will prepare sample apps later and add them to this pull request (if it's not merged by then) or create a new PR.