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/extension/src/extension_builder.rs

@@ -67,7 +67,7 @@ impl ExtensionBuilder {
     pub fn new(cache_dir: PathBuf) -> Self {
         Self {
             cache_dir,
-            http: http::client(),
+            http: http::client(None),
         }
     }