Add basic proxy settings (#11852)

Adding `proxy` keyword to configure proxy while using zed. After setting
the proxy, restart Zed to acctually use the proxy.

Example setting: 
```rust
"proxy" = "socks5://localhost:10808"
"proxy" = "http://127.0.0.1:10809"
```

Closes #9424, closes #9422, closes #8650, closes #5032, closes #6701,
closes #11890

Release Notes:

- Added settings to configure proxy in Zed

---------

Co-authored-by: Jason Lee <huacnlee@gmail.com>

crates/client/src/client.rs

@@ -114,10 +114,36 @@ impl Settings for ClientSettings {
     }
 }
 
+#[derive(Default, Clone, Serialize, Deserialize, JsonSchema)]
+pub struct ProxySettingsContent {
+    proxy: Option<String>,
+}
+
+#[derive(Deserialize, Default)]
+pub struct ProxySettings {
+    pub proxy: Option<String>,
+}
+
+impl Settings for ProxySettings {
+    const KEY: Option<&'static str> = None;
+
+    type FileContent = ProxySettingsContent;
+
+    fn load(sources: SettingsSources<Self::FileContent>, _: &mut AppContext) -> Result<Self> {
+        Ok(Self {
+            proxy: sources
+                .user
+                .and_then(|value| value.proxy.clone())
+                .or(sources.default.proxy.clone()),
+        })
+    }
+}
+
 pub fn init_settings(cx: &mut AppContext) {
     TelemetrySettings::register(cx);
     cx.update_global(|store: &mut SettingsStore, cx| {
         store.register_setting::<ClientSettings>(cx);
+        store.register_setting::<ProxySettings>(cx);
     });
 }
 
@@ -512,6 +538,7 @@ impl Client {
         let clock = Arc::new(clock::RealSystemClock);
         let http = Arc::new(HttpClientWithUrl::new(
             &ClientSettings::get_global(cx).server_url,
+            ProxySettings::get_global(cx).proxy.clone(),
         ));
         Self::new(clock, http.clone(), cx)
     }