Unify Substrate Bridge Pallet with Finality Verifier (#783)

* Add relevant storage items from Substrate pallet

* Add function for importing finalized headers to storage

* Remove unused getter functions

* Add GenesisConfig to pallet

* Add initialization extrinsic

* Add operational extrinsic

* Get existing finality verifier tests compiling again

* Add tests for pallet initialization

* Add tests related to pallet's operational status

* Update tests which were using `pallet-substrate-bridge`

* Add tests related to header imports

* Use wrapper function when init-ing some tests

* Add prefix to tests related to rate limiter

* Fix failed compilation related to GenesisConfig

* Add some documentation

* Change some extrinsics to be Operational

* Add public interface to pallet

* Implement runtime APIs for finality-verifier pallet

* Justify use of `expect` when importing headers

* Reject headers with forced changes

* Add weight to initialize extrinsic

* Remove TODO which will be addressed later

* Move succesful import log to correct location

* Expand proof for when `best_finalized` is fetched

* Move check for newer finalized blocks earlier in pipeline

* Rename `ConflictingFork` error to be more generic

* Only compute finality_target's hash once

* Add missing documentation to Runtime APIs

* Add TODO about using `set_id` from `ScheduledChange` digest