feat: Add ChatAnthropic integration

[davidmigloz]

Resolves #316

dependencies:
  langchain: {version}
  langchain_anthropic: {version}

final chatModel = ChatAnthropic(
  apiKey: apiKey,
  defaultOptions: const ChatAnthropicOptions(
    model: 'claude-3-5-sonnet-20240620',
    temperature: 0,
  ),
);