Attribute Macro handler 

#[handler]

Converts a synchronous or asynchronous function returning Result into a const_router handler.

The function may take either no request argument or one request argument:

• fn() -> Result<TResponse, TError>
• fn(TRequest) -> Result<TResponse, TError>
• async fn() -> Result<TResponse, TError>
• async fn(TRequest) -> Result<TResponse, TError>