Completions inconsistency

Every object gets their own handler accessible from the `OpenAI` class except for completions (both completions and chat completions). They each should get their own handler. This will also help the API match up better with the official APIs provided by OpenAI

Start by reading the OpenAI class and inspecting how the existing object handlers are exposed. Compare the current completions and chat-completions access patterns with the official API shape; done means each has its own handler and the public API is consistent with that structure.

Written by the indexing model from the issue text.