Use Msgpack encoded Bytes instead of JSON to transfer data 

If RPC call give bytes as response, it may not be JSON serializable in some languages.
So, we should use msgpack instead of JSON.

## Current schema
```graphql
request(method: String!, params: JSON, connection: Connection): JSON!
```

## Proposed Schema
```graphql
request(method: String!, params: Bytes!, connection: Connection): Bytes!
```

