feat: Add support for code execution in ChatGoogleGenerativeAI

[davidmigloz]

ChatGoogleGenerativeAI supports code execution, just set enableCodeExecution to true in the options.

final chatModel = ChatGoogleGenerativeAI(
  apiKey: apiKey,
  defaultOptions: ChatGoogleGenerativeAIOptions(
    model: 'gemini-1.5-flash',
    enableCodeExecution: true,
  ),
);
final res = await chatModel.invoke(
  PromptValue.string(
    'Calculate the fibonacci sequence up to 10 terms. '
        'Return only the last term without explanations.',
  ),
);

final text = res.output.content;
print(text); // 34

final executableCode = res.metadata['executable_code'] as String;
print(executableCode);

final codeExecutionResult = res.metadata['code_execution_result'] as Map<String, dynamic>;
print(codeExecutionResult);