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.

---------

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

crates/extension/src/extension_settings.rs

@@ -0,0 +1,39 @@
+use anyhow::Result;
+use collections::HashMap;
+use gpui::AppContext;
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+use settings::Settings;
+use std::sync::Arc;
+
+#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema)]
+pub struct ExtensionSettings {
+    #[serde(default)]
+    pub auto_update_extensions: HashMap<Arc<str>, bool>,
+}
+
+impl ExtensionSettings {
+    pub fn should_auto_update(&self, extension_id: &str) -> bool {
+        self.auto_update_extensions
+            .get(extension_id)
+            .copied()
+            .unwrap_or(true)
+    }
+}
+
+impl Settings for ExtensionSettings {
+    const KEY: Option<&'static str> = None;
+
+    type FileContent = Self;
+
+    fn load(
+        _default_value: &Self::FileContent,
+        user_values: &[&Self::FileContent],
+        _cx: &mut AppContext,
+    ) -> Result<Self>
+    where
+        Self: Sized,
+    {
+        Ok(user_values.get(0).copied().cloned().unwrap_or_default())
+    }
+}