How to send early hints

existing signature: `(req: Request) => Promise<Response>`

two options:

1. method on the request
   `req.sendEarlyHints(headers)`

1. part of the response
```ts
type EarlyHint = { headers: Headers; response: Promise<Response> };
type Handler = (req: Request) => Promise<Response> | Promise<EarlyHint>;
```