multiple handler routing / "Next" responder

It's there a way to pass a request to the next handler?
Something like
```rust
async fn handler() -> impl Responder {
if some_condition() {
Response::Ok()
} else {
Next
}
}
```

Start by reading the handler and Responder concepts shown in the issue, including how Response::Ok() is returned and how routing selects a handler. Determine whether a Next responder can be specified and what completion criteria the routing behavior would require; the issue names no files or tests to run.

Escrito por el modelo de indexación a partir del texto del issue.