feat(fromFetch): add Server-Sent Events supports

[kingcean]

Description:

Extends fromFetch function to enable Server-Sent Events.

import { fromFetch } from 'rxjs/fetch';

// URL_WITH_STREAM_MODE = "…";
// REQUEST_BODY = {…};

const data$ = fromFetch(URL_WITH_STREAM_MODE, {
  selector: 'sse'
  method: "POST",
  body: JSON.stringify(REQUEST_BODY),
  mode: "cors",
  credentials: "include",
  headers: {
    "Content-Type": "application/json",
    "Accept": "text/event-stream, application/json"
  },
});

data$.subscribe({
  next: result => console.log(result),
  complete: () => console.log('done')
});

Related issue:

Discussion #7584