Provide user agent when performing HTTP requests (#15470)

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/extension/src/extension_store.rs

@@ -243,10 +243,7 @@ impl ExtensionStore {
             extension_index: Default::default(),
             installed_dir,
             index_path,
-            builder: Arc::new(ExtensionBuilder::new(
-                ::http_client::client(http_client.proxy().cloned()),
-                build_dir,
-            )),
+            builder: Arc::new(ExtensionBuilder::new(http_client.clone(), build_dir)),
             outstanding_operations: Default::default(),
             modified_extensions: Default::default(),
             reload_complete_senders: Vec::new(),