grandpa: update to finality-grandpa v0.12.0 (#5853)

* grandpa: update to v0.12.0

* grandpa: fix tests

* grandpa: better validation of authority set invariants

* grandpa: avoid duplicating invalid authority list check

* grandpa: add missing doc

* grandpa: better validation of expect proofs

* grandpa: fix test compilation

* grandpa: fix tests

* grandpa: add test for AuthoritySet invariants

* grandpa: bump finality-grandpa to v0.12.0