grandpa-rpc don't share subscription manager, only executor (#7039)

* service builder: fix todo about jsonrpc Option workaround

* grandpa-rpc: only share executor instead of sub manager

* grandpa-rpc: fix compilation

* grandpa-rpc: rename to subscription_executor

* node/cli: remove another unused jsonrpc dependency

* grandpa: apply style fixes from code review

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