Attribute Macro handler 

#[handler]

Auto-registers an IRequestHandler implementation at compile time via inventory.

Place on impl IRequestHandler<T, R> for Handler blocks. The handler struct MUST implement Default.

#[handler]
#[async_trait]
impl IRequestHandler<HelloRequest, String> for HelloHandler {
    async fn handle(&mut self, _req: HelloRequest) -> Result<String> { ... }
}