Custom API request helper function

### What do you want and why?
Simplify requests to custom api routes that use blitz session middleware.

```ts
import { getAntiCSRFToken } from "blitz";

const antiCSRFToken = getAntiCSRFToken();
​
window.fetch("/api/custom", {
credentials: "include",
method: "POST",
body: JSON.stringify(data),
headers: { "Content-Type": "application/json", "anti-csrf": antiCSRFToken },
});
```

### Possible implementation(s)

```ts
import { fetchWithCredentials } from 'blitz';

fetchWithCredentials("/api/custom", {
method: "POST",
body: JSON.stringify(data),
headers: { "Content-Type": "application/json" },
});
```

Start by tracing getAntiCSRFToken and the custom /api/custom request shown in the issue, then inspect the existing Blitz request helpers and exports. Done means a reusable fetchWithCredentials helper can simplify authenticated custom API requests while preserving credentials and anti-CSRF handling.

Written by the indexing model from the issue text.