Add conversion from ParsedChatCompletion to ChatCompletion

[sternakt]

Why are these changes needed?

This PR introduces a _convert_to_chat_completion method that will convert ParsedChatCompletion objects into ChatCompletion objects. The ChatCompletion objects are picklable superclass of ParsedChatCompletion which contain all the fields that we use from the response in the client, so this approach enables caching of the responses generated while using response_format option.

Related issue number

Closes #133

Checks

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[ohdearquant]

seems like straightforward change, only comment is on the aesthetic.

self._oai_client.beta.chat.completions.parse(*args, **kwargs) is a bit long. but if that's the standard approach, then this is fine.

[marklysze]

Tested with response format and without, as well as with non-OpenAI client. Working as expected, thanks @sternakt.

Additionally, tested with 3.5 turbo and the exception was handled okay (as 3.5 turbo doesn't support json schemas).