Add Chat Completion Support

[jsissler]

Add support for Chat Completion dialogues using models such as gpt-4, gpt-4-0314, gpt-4-32k, gpt-4-32k-0314, gpt-3.5-turbo, gpt-3.5-turbo-0301. Implement and expose in a manner consistent with how prompt completions are currently implemented with example scenes and components accessible via the OpenAi Unity editor menus.