[feat] dartantic_ai builder

[csells]

to handle things like:

@Agent()
class TacAgent extends Agent {
  TacAgent(super.model);

  @Run()
  Future<TownAndCountry> run(String prompt) => _$TownAndCountryAgentRun(prompt);
}

void main() async {
  final result = await TacAgent('openai:gpt-4o').run('The windy city of the US of A.');
  print(result.output); // Output: TownAndCountry(town: Chicago, country: United States)
}