[feature request] Allow the users to do the serialization by their own for the payload in Direct Methods

This is from the issue #1584 where we notice there might be different behaviors across different JSON serialization and deserialization libraries when sending and receiving the direct method payloads. For instance, using Gson in Java and NewtonSoft in .NET. Therefore, it would be great to allow the users to do the serialization by their own, rather than giving the full control to the SDK internally.