Add Unchecked Header Writes to Bridge Pallet (#672)

* Add method for unchecked header imports

* Update docs for importing unchecked headers

* Import unchecked headers in HeaderChain implementation

* Fix Clippy warnings

* Move unchecked header import out of Verifier struct

* Clean up unchecked import tests

* Change HeaderChain API to accept iterator of headers

* Use chains of headers in tests

* Remove unused Result return type when appending finalized headers

* Add test which shows that genesis changes are not enacted

* Use initial header's hash for unchecked authority set changes

* Appease Clippy

* Check ancestry before making unchecked writes

* Fix typo

* Fix Clippy warning

* Add note about `ancestry_proof` structure

* Use best hash storage item directly

Co-authored-by: Svyatoslav Nikolsky <[email protected]>