Cleaner GRANDPA RPC API for proving finality (#7339)

* grandpa: persist block number for last block of authority set

* grandpa: fix authority_set_changes field in tests

* grandpa: fix date on copyright notice

* grandpa-rpc: implement cleaner api for prove finality rpc

* grandpa-rpc: replace the old prove_finality with the new one

* grandpa: undo accidental whitespace change

* grandpa-rpc: start work on redo of the finality_proof RPC API

* grandpa: manual impl of Decode for AuthoritySet

* grandpa: add comment about appending changes for forced changes

* grandpa: flip order in set changes, tidy up some comments

* grandpa: update some of the doc comments

* grandpa: store authority set changes when applying forced changes

* grandpa: simplify finality_proof.rs

* grandpa: move checks and extend tests in finality_proof

* grandpa: address first set of review comments

* grandpa: check that set changes have well-defined start

* grandpa: rework prove_finality and assocated tests

* grandpa: make AuthoritySetChanges tuple struct

* grandpa: add assertions for tracking auth set changes

* grandpa: remove StorageAndProofProvider trait

* grandpa: return more informative results for unexpected input to RPC

* grandpa: tiny tweak to error msg

* grandpa: fix tests

* grandpa: add error specific to finality_proof

* grandpa: fix review comments

* grandpa: proper migration to new AuthoritySet

* grandpa: fix long lines

* grandpa: fix unused warning after merge

Co-authored-by: André Silva <andrerfosilva@gmail.com>