feat: Support responseSchema in ChatFirebaseVertexAI

[davidmigloz]

Fixed some breaking changes they introduced in the first stable release.

Now you can provide a responseSchema for structured outputs:

final options = ChatFirebaseVertexAIOptions(
  //...
  responseSchema: {
    'type': 'object',
    'properties': {
      'answer': {
        'type': 'string',
        'description': 'The answer to the question being asked',
      },
      'sources': {
        'type': 'array',
        'items': {'type': 'string'},
        'description': 'The sources used to answer the question',
      },
    },
    'required': ['answer', 'sources'],
  },
);