zed_extension_api: Add fetch (#13716)

This PR adds a new `fetch` function to the `zed_extension_api` to allow
fetching a URL through the Wasm host.

Currently we only support GET requests and return the response body as a
string.

Release Notes:

- N/A

crates/extension_api/wit/since_v0.0.7/extension.wit

@@ -2,6 +2,7 @@ package zed:extension;
 
 world extension {
     import github;
+    import http-client;
     import platform;
     import nodejs;
 

crates/extension_api/wit/since_v0.0.7/http-client.wit

@@ -0,0 +1,16 @@
+interface http-client {
+    /// An HTTP request.
+    record http-request {
+        /// The URL to which the request should be made.
+        url: string,
+    }
+
+    /// An HTTP response.
+    record http-response {
+        /// The response body.
+        body: string,
+    }
+
+    /// Performs an HTTP request and returns the response.
+    fetch: func(req: http-request) -> result<http-response, string>;
+}