Move justification code to primitives crate (#640)

* Move justification module to header-chain primitives crate

* Get justification module compiling in new location

* Get justification module tests compiling

* Use justification code from `header-chain` crate

Mostly compiles, having issues with std/test feature flags across crates.

* Move some code around

* Move justification tests to integration testing crate

* Add `test-utils` crate

* Remove tests and test-helper module from justification code

* Use `test-utils` in Substrate bridge pallet tests

* Remove `sp-keyring` related code from `pallet-substrate-bridge`

* Remove `helpers` module from `pallet-substrate-bridge`

* Add some documentation

* Add more documentation

* Fix typo

Co-authored-by: Tomasz Drwięga <[email protected]>

Co-authored-by: Tomasz Drwięga <[email protected]>