ValidatorSet reporting (#4208)

* remove register_account_provider

* build rpc module

* new dummy client

* common EngineSigner struct

* from -> into

* initial report via call

* separate safe from regular contract

* transact_contract

* fix build

* return Signature, docs

* add sign method to some engines

* add safeContract spec

* update specs to new contracts

* use AuthorityRound for contract spec

* add more reporting

* add reporting test

* use gas floor

* shorter