RPC integration

## Issue Summary
From design doc:
> Gossamer already has a [`GrandpaModule`](https://github.com/ChainSafe/gossamer/blob/2eff00475ac1234ac1701f596808f93b3bf0bd46/dot/rpc/modules/grandpa.go#L15) to prove finality, retrieve round state, and a [`GrandpaJustificationListener`](https://github.com/ChainSafe/gossamer/blob/db64e1b48b5b0c3d234be0099aa176d298693bd8/dot/rpc/subscription/listeners.go#L439) to subscribe to justifications. Changes will need to be made to support the new GRANDPA client. This can be be deprioritized given that we do not need this immediately given our current milestones.

## Acceptance Criteria
- Revise current GRANDPA RPC implementation to fetch from new GRANDPA client.
- Unit Tests

## Previous Research
### Issue summary
- Integrate existing RPC calls associated with GRANDPA to new GRANDPA service.

### Other information and links

### Estimate
1 week