feat: Add support for JsonOutputParser

[davidmigloz]

Output parser that returns the output of the previous Runnable as a JSON Map.

Example:

final model = ChatOpenAI(
  apiKey: openAiApiKey,
  defaultOptions: const ChatOpenAIOptions(
    responseFormat: ChatOpenAIResponseFormat(
      type: ChatOpenAIResponseFormatType.jsonObject,
    ),
  ),
);
final parser = JsonOutputParser<ChatResult>();

final chain = model.pipe(parser);

final stream = chain.stream(
  PromptValue.string(
    'Output a list of the countries france, spain and japan and their '
    'populations in JSON format. Use a dict with an outer key of '
    '"countries" which contains a list of countries. '
    'Each country should have the key "name" and "population"',
  ),
);
await stream.forEach((final chunk) => print('$chunk|'));
// {}|
// {countries: []}|
// {countries: [{}]}|
// {countries: [{name: }]}|
// {countries: [{name: France}]}|
// {countries: [{name: France, population: 670}]}|
// {countries: [{name: France, population: 670760}]}|
// {countries: [{name: France, population: 67076000}]}|
// {countries: [{name: France, population: 67076000}, {}]}|
// {countries: [{name: France, population: 67076000}, {name: }]}|
// {countries: [{name: France, population: 67076000}, {name: Spain}]}|
// {countries: [{name: France, population: 67076000}, {name: Spain, population: 467}]}|
// {countries: [{name: France, population: 67076000}, {name: Spain, population: 467237}]}|
// {countries: [{name: France, population: 67076000}, {name: Spain, population: 46723749}]}|
// {countries: [{name: France, population: 67076000}, {name: Spain, population: 46723749}, {}]}|
// {countries: [{name: France, population: 67076000}, {name: Spain, population: 46723749}, {name: }]}|
// {countries: [{name: France, population: 67076000}, {name: Spain, population: 46723749}, {name: Japan}]}|
// {countries: [{name: France, population: 67076000}, {name: Spain, population: 46723749}, {name: Japan, population: 126}]}|
// {countries: [{name: France, population: 67076000}, {name: Spain, population: 46723749}, {name: Japan, population: 126476}]}|
// {countries: [{name: France, population: 67076000}, {name: Spain, population: 46723749}, {name: Japan, population: 126476461}]}|