[Feature Request] Do not clobber user payload codec in OpenAI plugin

[cretz]

Describe the solution you'd like

OpenAI plugin replaces the entire data converter instead of just the payload converter. Also, per #1083 (review), we should consider erroring if the data converter is provided with a non-default payload converter (since we are overriding it). Or we could error only if the user-provided payload converter is not what we need (Pydantic with a setting set). There could be discussions on advanced user payload converter customization on top of the one OpenAI plugin sets, but that can wait until needed.