Please add response_format to create_completion

**Is your feature request related to a problem? Please describe.**
The function create_chat_completion has an option for a response_format parameter which can be used to specify things like JSON output with an optional schema. This is great. However, this option is completely missing from create_completion.

Problem is, what am I supposed to do if I need a chat format that isn't supported by create_chat_completion? I don't mind manually creating the prompt in these cases, but not being able to ensure JSON output is a serious limitation.

**Describe the solution you'd like**
Support the response_format parameter in create_completion the same as it is in create_chat_completion.