Add a `default_open_ai_model` setting for the assistant (#2876)

[This PR has been sitting around for a
bit](https://github.com/zed-industries/zed/pull/2845). I received a bit
of mixed opinions from the team on how this setting should work, if it
should use the full model names or some simpler form of it, etc. I went
ahead and made the decision to do the following:

- Use the full model names in settings - ex: `gpt-4-0613`
- Default to `gpt-4-0613` when no setting is present
- Save the full model names in the conversation history files (this is
how it was prior) - ex: `gpt-4-0613`
- Display the shortened model names in the assistant - ex: `gpt-4`
- Not worry about adding an option to add custom models (can add in a
follow-up PR)
- Not query what models are available to the user via their api key (can
add in a follow-up PR)

Release Notes:

- Added a `default_open_ai_model` setting for the assistant (defaults to
`gpt-4-0613`).

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Co-authored-by: Mikayla <mikayla@zed.dev>