Combine zeta and zeta2 edit prediction providers

[maxbrunsfeld]

We've realized that a lot of the logic within an EditPredictionProvider is not specific to a particular edit prediction model / service. Rather, it is just the generic state management required to perform edit predictions at all in Zed. We want to move to a setup where there's one "built-in" edit prediction provider in Zed, which can be pointed at different edit prediction models. The only logic that is different for different models is how we construct the prompt, send the request, and parse the output.

This PR also changes the behavior of the staff-only zeta2 feature flag so that in only gates your ability to use Zeta2, but you can still use your local settings file to choose between different edit prediction models/services: zeta1, zeta2, and sweep.

This PR also makes zeta1's outcome reporting and prediction-rating features work with all prediction models, not just zeta1.

To do:

• remove duplicated logic around sending cloud requests between zeta1 and zeta2
• port the outcome reporting logic from zeta to zeta2.
• get the "rate completions" modal working again (with all EP models)
  • display edit prediction diff
  • show edit history events
• move the "zeta2" logic into the edit_predictions crate - it will no longer be specific to the zeta2 model
• remove the original zeta crate.

Release Notes:

• N/A