Async config loading (#18022)

Parts of config will come from executor. Prepare for that by making
config loading methods async.

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

@@ -716,6 +716,7 @@ mod tests {
         match ConfigBuilder::default().build().await {
             Ok(config) => config,
             Err(_) => Config::load_default_with_cli_overrides(Vec::new())
+                .await
                 .expect("default config should load"),
         }
     }