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/gpui/src/app.rs

@@ -115,7 +115,7 @@ impl App {
         Self(AppContext::new(
             current_platform(),
             Arc::new(()),
-            http::client(),
+            http::client(None),
         ))
     }
 
@@ -651,6 +651,11 @@ impl AppContext {
         self.platform.local_timezone()
     }
 
+    /// Updates the http client assigned to GPUI
+    pub fn update_http_client(&mut self, new_client: Arc<dyn HttpClient>) {
+        self.http_client = new_client;
+    }
+
     /// Returns the http client assigned to GPUI
     pub fn http_client(&self) -> Arc<dyn HttpClient> {
         self.http_client.clone()