Function make_post_handler 

fn make_post_handler<P, F, Fut>(
    f: F,
) -> Box<dyn Fn(Extension<Arc<Gateway>>, Value) -> Pin<Box<dyn Future<Output = Result<Json<Value>, GatewayError>> + Send>> + Send + Sync>
where
    P: DeserializeOwned + Send + 'static,
    F: Fn(Extension<Arc<Gateway>>, Json<P>) -> Fut + Clone + Send + Sync + 'static,
    Fut: Future<Output = Result<Json<Value>, GatewayError>> + Send + 'static,

Creates a POST handler for the Iroh endpoint by wrapping it in a closure.