To show a `+` button to add a new field in a node’s toolbar we'll want to add an `actions` prop to the Node interface. 

Let's also add a story book to keep that up to date.

  /**
   * Optional actions to display in the node header.
   */
  actions?: React.ReactNode;

The node header:
https://github.com/mongodb-js/diagramming/blob/b96912ef896f515afaafdfa7e1f07f7d4e2a0d90/src/components/node/node.tsx#L180