[Enhancement] requestPermission() should return an observable or promise

[CramericaIndustries]

requestPermission() should return an observable or promise so you can send a notification as soon as the user approved the use of the Notification API.

this.pushNotificationsService.requestPermission().subscribe(permission => {
   if(permission === "granted") {
      this.pushNotificationsService.create("title",{...}).subscribe(...);
   }
});

Should be easy to implement as the underlying API call Notification.requestPermission() already returns a promise. @see https://developer.mozilla.org/en-US/docs/Web/API/Notification/requestPermission