Support reference answers in the OpenAIMessagesDataset

[junya-takayama]

Enable setting the references field of ChatInstance from OpenAIMessagesDataset as well.

Specifically, references will be set using the following mechanism:

• If an arbitrary key name is passed to references_key, the value stored under that key in the JSON object is used as references.
  • The value may be either a str (single reference) or a list[str] (multiple references). Any other type results in an error.
• If drop_if_last_from_assistant is set to true, the last assistant utterance (the one that was dropped) is set as references.
  • However, if references_key is set, it takes precedence.

[h-asano]

LGTM！