This adds the possibility to override MSW mocks with a dynamic delay
using a function that returns a number, making API calls more similar to
real-world ones, where responses can potentially come back at different
times:

```js
{
  // ... other options ...
  override: {
    mock: {
      delay: () => { ... }
    }
  }
}
```

This change is backwards compatible and the previous default value has
been maintained.