Re: [whatwg/fetch] Trailer support in the API (#981)

Howdy all,

I work at Fastly on their JavaScript runtime and SDK 👋 
Fastly are interested in implementing HTTP Trailer support in order to support gRPC over HTTP/2.
I would like to implement the HTTP Trailer support in a standard compatible way, and would be happy to work with folks on bringing this functionality into the Fetch Standard.

What I am thinking as a potential solution is:

Updating the `Response` interface to include a new `trailers` method which returns a `Promise<Headers>` instance:
```webidl
[SameObject] Promise<Headers> trailers();
```

Updating the `ResponseInit` interface to include a new `trailers` field which implements the `HeadersInit` interface
```webidl
[HeadersInit](https://fetch.spec.whatwg.org/#typedefdef-headersinit) trailers;
```

And the same for both `Request` and `RequestInit`.

Which in an application context would look like this:
```js
// Service-Worker API based example of reading a request trailer
// and responding with a response which contains the same trailer

addEventListener('fetch', event => event.respondWith(app(event)));

async function app(event) {
  const request = event.request;
 // Resolves when all trailer fields have been received and returns an instance of Headers
 const incomingTrailers = await request.trailers();
 const animal = incomingTrailers.get('animal');
 const response = new Response('I am a body', {
  headers: {
   'Trailer': 'animal'
  },
  trailers: {
   animal: animal
 });
 return response;
}
```


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