Support payload field inside Droppable that is sent to DropResult

[harisrozajac]

Hey I love this library!

In this PR I'm adding a payload that can be passed to Draggable component and then show up in DropResult when onDragEnd is triggered. I have two applications with shared DragDropContext and want to share some data between them by dragging and dropping items. This solution will simplify some custom data sharing and not having to keep a separate state and then doing lookups through draggableId.

Example usage:

<Draggable
     draggableId="id"
     index={0}
     payload={"some data"}
>

The payload would then show up in DropResult like this:

{
    "draggableId": "id",
    "type": "DEFAULT",
    "source": {
        "index": 0,
        "droppableId": "list-app-1",
        "payload": "some data"
    },
    "reason": "DROP",
    "mode": "FLUID",
    "destination": {
        "droppableId": "list-app-2",
        "index": 1
    },
    "combine": null
}

Fixes: #1806