WebSockets: string should be used instead of `SubscribeArgs`

For some reason tendermint use different argument for Subscription in WebSocket and RPC.

In WS handling, query is "wrapped": https://github.com/tendermint/tendermint/blob/9379bc92fdbe344bbfa82b05c1854029edc56e46/rpc/jsonrpc/client/ws_client.go#L527-L530

Optimint needs to use similar logic for compatibility.