Allow for json mode

[LukasWallrich]

It would be great to use JSON mode, yet even with the development version, I don't see how to pass that parameter. Could that maybe be added?

https://platform.openai.com/docs/guides/text-generation/json-mode

[irudnyts]

@LukasWallrich, this feature was actually in the development version. But there are a couple of pitfalls that require care. Please see an example below with commented points:

remotes::install_github("irudnyts/openai", ref = "r6")
library(openai)

client <- OpenAI()
completion <- client$chat$completions$create(
    model = "gpt-3.5-turbo-1106", # ☝️ must be either "gpt-4-1106-preview" or "gpt-3.5-turbo-1106"
    messages = list(list("role" = "user", "content" = "What's up? Answer in JSON.")), # ☝️ You must include something like the answer should be in JSON
    response_format = list("type" = "json_object")
)

completion$choices[[1]]$message$content