grandpa-rpc: allow proving finality of blocks from latest authority set (#8585)

* grandpa: use new latest stored justification in prove_finality

* grandpa: include end in range in FinalityProof::unknown_headers

* grandpa: typo in comment

* grandpa: remove ProvableJustification

* grandpa: revert unnecessary changes

* grandpa: extend AuthoritySetChangeId and cleanup get_set_id

* grandpa: move check_finality_proof to the test module

* grandpa: warn on missing authority set changes data

* grandpa: add missing use statement

* grandpa: simplify finality_proof tests

* grandpa: additional tests for finality_proof

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