Could we add option to send pure string for Client.rpc api?

[xinatcg]

https://github.com/heroiclabs/nakama-js/blob/ea29ed4b81b91364a1338bd290450b3b98daeb4d/packages/nakama-js/client.ts#L1742C3-L1754C4

We use base64 to encode the payload as a pure string and the Unity client supports sending a pure string, but here it always JSON.stringify, so it cannot send a pure string.

The socket.rpc supports a pure string. Could we add an option to skip the JSON.stringify and send a pure string here?