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 <123550+andresilva@users.noreply.github.com>

* add `prove_finality_proof` to `FinalityProofProvider`

* fix some and impl Clone for FinalityProofProvider

* improve by suggestions

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Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>