* introduce polkadot-node-primitives

* guide: change statement distribution message types

* guide: remove variant from `CandidateSelectionMessage`

* add a few more message types

* add TODOs

* Almost all messages

* NewBackedCandidate notification

* Formatting

* Use AttestedCandidate as BackedCandidate

* Update node/primitives/src/lib.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Fix the tests

* Bring in types from #1242

* Adds network bridge messages

* More message types from doc

* use fn pointer type

* Fixes from the review

* Add missing Runtime subsystem message

* rename to CandidateValidationMessage and fix tests

Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>
Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>