feat: Add support for Runnable.mapInputStream

[davidmigloz]

A RunnableMapInputStream allows you to map the input stream to a different stream of values.

Example:

final model = ChatOpenAI(
  apiKey: openAiApiKey,
  defaultOptions: ChatOpenAIOptions(
    responseFormat: ChatOpenAIResponseFormat(
      type: ChatOpenAIResponseFormatType.jsonObject,
    ),
  ),
);
final parser = JsonOutputParser<ChatResult>();
final mapper = Runnable.mapInputStream((Stream<Map<String, dynamic>> inputStream) async* {
  yield await inputStream.last;
});
final chain = model.pipe(parser).pipe(mapper);
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: [{name: France, population: 65273511}, {name: Spain, population: 46754778}, {name: Japan, population: 126476461}]}|