Add experimental exec server URL handling (#15196)

Add a config and attempt to start the server.

codex-rs/app-server/src/fs_api.rs

@@ -34,7 +34,7 @@ pub(crate) struct FsApi {
 impl Default for FsApi {
     fn default() -> Self {
         Self {
-            file_system: Arc::new(Environment.get_filesystem()),
+            file_system: Arc::new(Environment::default().get_filesystem()),
         }
     }
 }