Add multipart chunked flag

Fixes https://github.com/request/request/issues/1243

That's just to show up the idea from my last comment. Basically I'm bringing back the code from v2.46 and I'm introducing a flag so that both implementations can exist together.

I'm proposing this syntax for non chunked/streamed multipart/related bodies

```js
multipart: {
  data: []
}
// in place of the default one
multipart: []
```

Obviously that's ugly, but I can't think of anything more reasonable at this point. Once we agree on the syntax I'll add tests for it.