Add streamCallbacks as an Optional Argument and Implement Stream Handling Logic : Fixes #322

[Sufiyan-MSA]

Added streamCallbacks as an optional argument to generateContentStream.

This allows users to avoid dealing directly with Node.js streams and instead receive the streamed text via simple callback functions.

Example usage:

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

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