Add feature flag to use cloud.zed.dev instead of llm.zed.dev (#34076)

This PR adds a new `zed-cloud` feature flag that can be used to send
traffic to `cloud.zed.dev` instead of `llm.zed.dev`.

This is just so Zed staff can test the new infrastructure. When we're
ready for prime-time we'll reroute traffic on the server.

Release Notes:

- N/A

crates/http_client/src/http_client.rs

@@ -226,10 +226,21 @@ impl HttpClientWithUrl {
     }
 
     /// Builds a Zed LLM URL using the given path.
-    pub fn build_zed_llm_url(&self, path: &str, query: &[(&str, &str)]) -> Result<Url> {
+    pub fn build_zed_llm_url(
+        &self,
+        path: &str,
+        query: &[(&str, &str)],
+        use_cloud: bool,
+    ) -> Result<Url> {
         let base_url = self.base_url();
         let base_api_url = match base_url.as_ref() {
-            "https://zed.dev" => "https://llm.zed.dev",
+            "https://zed.dev" => {
+                if use_cloud {
+                    "https://cloud.zed.dev"
+                } else {
+                    "https://llm.zed.dev"
+                }
+            }
             "https://staging.zed.dev" => "https://llm-staging.zed.dev",
             "http://localhost:3000" => "http://localhost:8787",
             other => other,