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/semantic_index/examples/index.rs

@@ -26,7 +26,7 @@ fn main() {
         });
 
         let clock = Arc::new(FakeSystemClock::default());
-        let http = Arc::new(HttpClientWithUrl::new("http://localhost:11434"));
+        let http = Arc::new(HttpClientWithUrl::new("http://localhost:11434", None));
 
         let client = client::Client::new(clock, http.clone(), cx);
         Client::set_global(client.clone(), cx);