collator-protocol: add message authentication (#2635)

* collator: authenticate collator protocol messages

* fix tests compilation

* node: verify collator protocol signatures in tests

* collator: fix tests

* implementers-guide: update CollatorProtocol messages

* collator: add test for verification of collator protocol signatures

* node: remove fixmes

* node: remove signature from advertisecollation message

* node: add magic constant to Declare message signature payload