feat: add generateAnswer() for AQA support

[Kartikayy007]

Resolves #80

Feature: AQA Support via generateAnswer()

What's changed

Implements the requested AQA (Attributed Question Answering) support by adding:

• New generateAnswer() method in GenerativeModel class
• Task.GENERATE_ANSWER enum value
• Defined GenerateAnswerRequest and GenerateAnswerResponse interfaces
• Updated tests

Testing

• Verified against AQA API endpoints
• Added comprehensive test cases

Example Usage

import {GoogleGenerativeAI} from '@google/generative-ai';

const genAI = new GoogleGenerativeAI(API_KEY);
const model = genAI.getGenerativeModel({model: 'models/aqa'});

try {
  const result = await model.generateAnswer({
    input: "What causes seasons?",
    sources: [{
      title: "NASA Earth Facts",
      content: "Earth's axial tilt causes seasonal variations..."
    }],
    temperature: 0.7
  });
  
  console.log('Answer:', result.answer);
  console.log('Confidence:', result.confidenceScore);
} catch (err) {
  console.error('AQA Error:', err);
}

[Kartikayy007]

@IvanLH @hsubox76 Could I get your review on this AQA implementation when you have time?

[Kartikayy007]

hey @hkt74 do this feature implementation seem fine?