Simulate connect delay

Nobody has claimed this yet.

Hey,

thanks for the crate.

I'm building a [link checker](https://github.com/hello-rust/lychee) and I need a way to simulate a connect delay.
Here's a little snippet to show what I tried:

```rust
#[tokio::test]
async fn test_timeout() {
let mock_server = MockServer::start().await;
let delay = Duration::from_secs(30);
let template = ResponseTemplate::new(200).set_delay(delay.clone());
Mock::given(method("GET"))
.respond_with(template)
.mount(&mock_server)
.await;

let res = get_checker(false, HeaderMap::new())
.check(&Url::parse(&mock_server.uri()).unwrap())
.await;
assert!(matches!(res, Status::Timeout));
}
```

`get_checker` is just a wrapper around reqwest.

This didn't work. It's obvious, in hindsight, because I was mixing up the _response_ delay with a _connect_ delay.
Any way I could test that right now? Maybe there could be another parameter `set_connect_delay` for that?

The issue names MockServer and ResponseTemplate::set_delay but no repository file or test path. Start by tracing how set_delay is implemented and how MockServer accepts connections, then compare the behavior with the shown reqwest timeout test. Done means a connect-delay capability is exposed and the timeout scenario can be tested reliably.

Written by the indexing model from the issue text.