feat: improve FinalityProofProvider api (#13374)

* feat: improve prove_finality api and export it

* fmt

* fix

* improve prove_finality and kept private

* Update client/finality-grandpa/src/finality_proof.rs

Co-authored-by: André Silva <[email protected]>

* add `prove_finality_proof` to `FinalityProofProvider`

* fix some and impl Clone for FinalityProofProvider

* improve by suggestions

---------

Co-authored-by: André Silva <[email protected]>