Allow overriding of edit_prediction_provider in project settings

[kradalby]

Hi!

I looked for issues opened on this, but might have missed it.

I have a couple of different code bases where we care cleared to use various AI models for various things, and I would like to override edit_prediction_provider on a per project bases since I like using the Zed provider for some projects while others have stricter requirements and only Copilot is approved.

When doing some local testing, I was not able to get this working, and reading the configuring Zed, it is a bit unclear if this option falls into that group or not.

Hope this make sense, let me know if you need more elaboration!