Stop Importing Full Header Chain (#707)

* Make AncestryProof type more flexible

* Only import single finalized header instead of a chain

* Fix unchecked header import tests

* Add option for limiting ancestry proof size

* Update finality verifier Config in runtimes

* Update some documentation

* Fix Clippy warning

* Allow AncestryChecker to return proof size

Stops us from abusing the `Size` trait

* Remove Size impl for Vec<T>

* Remove size contraints for ancestry proofs

With different proof types its unclear how to "size" should be interpreted,
so we remove this requirement all together to avoid confusion.