feat(output-parsers): Add streaming support to OutputFunctionsParsers

[davidmigloz]

Example:

const function = ChatFunction(
  name: 'joke',
  description: 'A joke',
  parameters: {
    'type': 'object',
    'properties': {
      'setup': {
        'type': 'string',
        'description': 'The setup for the joke',
      },
      'punchline': {
        'type': 'string',
        'description': 'The punchline to the joke',
      },
    },
    'required': ['location', 'punchline'],
  },
);
final promptTemplate = ChatPromptTemplate.fromTemplate(
  'tell me a long joke about {foo}',
);
final chat = ChatOpenAI(apiKey: openaiApiKey, temperature: 0).bind(
  ChatOpenAIOptions(
    functions: const [function],
    functionCall: ChatFunctionCall.forced(functionName: 'joke'),
  ),
);
final jsonOutputParser = JsonOutputFunctionsParser();
final chain = promptTemplate.pipe(chat).pipe(jsonOutputParser);

final stream = chain.stream({'foo': 'bears'});

await for (final res in stream) {
  print(res);
}

will print:

{}
{setup: }
{setup: Why}
{setup: Why don}
{setup: Why don't}
{setup: Why don't bears}
{setup: Why don't bears like}
{setup: Why don't bears like fast}
{setup: Why don't bears like fast food}
{setup: Why don't bears like fast food?, punchline: }
{setup: Why don't bears like fast food?, punchline: Because}
{setup: Why don't bears like fast food?, punchline: Because they}
{setup: Why don't bears like fast food?, punchline: Because they can}
{setup: Why don't bears like fast food?, punchline: Because they can't}
{setup: Why don't bears like fast food?, punchline: Because they can't catch}
{setup: Why don't bears like fast food?, punchline: Because they can't catch it}
{setup: Why don't bears like fast food?, punchline: Because they can't catch it!}