Change auto-update to optional, default to null

[mitchellh]

Change the auto-update config¹ to an optional ?AutoUpdate and default it to "null".

The behavior when "null" (unset) is that we defer to the Sparkle default behavior. This will allow users to override this using standard defaults. I think this will also incidentally change it so that on first run Sparkle asks the user for permission to check for updates instead of us pinging right away (which should alleviate some users privacy concerns although we don't track that data).

This will also make it so the checkbox during update dialogs work.

Footnotes

1. https://ghostty.org/docs/config/reference#auto-update ↩