Add FallbackModel support

[sydney-runkle]

This PR adds a FallbackModel to the models folder that allows for delegation of a task to a new model if the current model fails with a 4xx or 5xx response. See the docs for a detailed example.

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Fix #516

This is one of the items on our roadmap as well.

In a follow up PR:

• Rename ModelStatusError and rethink our error structure internally (before the next release)
• Consider errors with no status code (connection errors)

[dmontagu]

This looks great. Let's confirm the new exception names with Samuel tomorrow morning then merge.

[sydney-runkle]

Thanks @dmontagu for picking up the rest of the changes here :)

[Kludex]

This PR is mine now.

[github-actions]

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[Kludex]

1. We should be able to pass a callable that determines if we should fallback to the next model, or a tuple of exception types.
2. We need to subclass ExceptionGroup.