: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/Cargo.toml

@@ -10,6 +10,10 @@ rust-embed = "8.5.0"
 path = "../pmoserver"
 optional = true
 
+[dependencies.async-trait]
+optional = true
+version = "0.1"
+
 [features]
 default = []
-pmoserver = ["dep:pmoserver"]
+pmoserver = ["dep:pmoserver", "dep:async-trait"]