Add auto-update system for extensions (#9890)

* [x] auto update extensions on startup
* [ ] add a manual way of updating all?
* [x] add a way to opt out of auto-updates for a particular extension

We don't believe that there should be any background polling for
extension auto-updates, because it could be disruptive to the user.

Release Notes:

- Added an auto-update system for extensions.

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Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>