Enhance generateContentStream with streamCallbacks support. Fixes #322

[cestercian]

• Adds streamCallbacks as an optional argument to generateContentStream.
• Enables users to receive text responses via callbacks without handling Node.js streams.
• Includes test cases for the new functionality.

Example Usage:

await model.generateContentStream("Tell me a joke", {}, {
   onData: (chunk: string) => process.stdout.write(chunk),  
   onDone: (fullText: string) => console.log("\nFinal output:", fullText),
});

[demoncoder-crypto]

I am working on this issue let me know if its up to expectation