feat(chat-models): Add streaming support to ChatOpenAI

[davidmigloz]

Relates to #4

Example:

final promptTemplate = ChatPromptTemplate.fromPromptMessages([
  SystemChatMessagePromptTemplate.fromTemplate(
    'You are a helpful assistant that replies only with numbers '
    'in order without any spaces or commas',
  ),
  HumanChatMessagePromptTemplate.fromTemplate(
    'List the numbers from 1 to {max_num}',
  ),
]);

final chat = ChatOpenAI(apiKey: openaiApiKey);
const stringOutputParser = StringOutputParser<ChatMessage>();
final chain = promptTemplate.pipe(chat).pipe(stringOutputParser);

final stream = chain.stream({'max_num': '9'});

String content = '';
await for (final res in stream) {
  content += res;
  print(content);
}

will print:

123
123456
123456789