Getting logprobs for OpenAI models

[bglearning]

Is your feature request related to a problem? Please describe.

Afaict, there isn't a way to get the logprobs from OpenAI models (at least the ones that support it). It is lost during conversion to ChatMessage.

Could be good to have it in for things like confidence proxy (though crude) or autocomplete suggestions.

[sjrl]

Hey @bglearning could you provide an example config of an OpenAIChatGenerator that would return logprobs?