grandpa: make the VotingRule API async (#8101)

* grandpa: make the VotingRule api async

* grandpa: add docs to VotingRuleResult

* grandpa: formatting

* grandpa: use async blocks

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* grandpa: expose VotingRuleResult

* grandpa: revert some broken changes to async syntax

* grandpa: use finality-grandpa v0.14.0

* grandpa: bump impl_version

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>