Callback for when fields are added to the stage. Good as a catch-all action for anything that needs to happen before a field is added to the stage. Possible to modify the field's configuration before it's appended by returning a fieldData object eg:

        "type": "number",
        "label": "Big Numbers",
        "className": "form-control",
        "name": "number-1532639192296"


const options = {
  onFieldAdd: function(fieldId) {
    const currentFieldId = fieldId