:sparkles!: add async-trait dependency for WebAppExt trait

- Add `async_trait` as an optional依赖ency in pmoapp/Cargo.toml
- Enable usage of `#[async_trait]` on WebAppExt trait and its impl for Server
- Guard async-related code with `#[cfg(feature = "pmoserver")]`

pmoapp/src/pmoserver_impl.rs

@@ -27,10 +27,14 @@
 //! # }
 //! ```
 
+#[cfg(feature = "pmoserver")]
+use async_trait::async_trait;
 use crate::WebAppExt;
 use pmoserver::Server;
 use rust_embed::RustEmbed;
 
+#[cfg(feature = "pmoserver")]
+#[async_trait]
 impl WebAppExt for Server {
     async fn add_webapp<W>(&mut self, path: &str)
     where