Adds streamCallbacks as an optional argument. Fixes #322

[hkirat]

Added streamCallbacks as an optional argument to generateContentStream.
This would allow users to not deal with node.js streams and simply get back the text response in callbacks.

   await model.generateContentStream("What is 2 + 2", {}, {
       onData(chunk: string) => console.log(chunk),
      onDone(fullText: string) => console.log(fullText)
   });

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