feat(llms): Add streaming support to OpenAI

[davidmigloz]

Relates to #4

Example:

final promptTemplate = PromptTemplate.fromTemplate(
  'List the numbers from 1 to {max_num} in order without any spaces or commas',
);
final llm = OpenAI(apiKey: openaiApiKey);
const stringOutputParser = StringOutputParser<String>();
final chain = promptTemplate.pipe(llm).pipe(stringOutputParser);

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

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

will print:

123
12345
1234567
123456789