Add OS specific settings (#35756)

Release Notes:

- Settings can now be configured per operating system with the new
top-level fields: `"macos"`/`"windows"`/`"linux"`. These will override
user level settings, but are lower precedence than _release channel_
settings.

crates/auto_update/src/auto_update.rs

@@ -134,10 +134,15 @@ impl Settings for AutoUpdateSetting {
     type FileContent = Option<AutoUpdateSettingContent>;
 
     fn load(sources: SettingsSources<Self::FileContent>, _: &mut App) -> Result<Self> {
-        let auto_update = [sources.server, sources.release_channel, sources.user]
-            .into_iter()
-            .find_map(|value| value.copied().flatten())
-            .unwrap_or(sources.default.ok_or_else(Self::missing_default)?);
+        let auto_update = [
+            sources.server,
+            sources.release_channel,
+            sources.operating_system,
+            sources.user,
+        ]
+        .into_iter()
+        .find_map(|value| value.copied().flatten())
+        .unwrap_or(sources.default.ok_or_else(Self::missing_default)?);
 
         Ok(Self(auto_update.0))
     }