Add a mechanism to add extraCols (from reporting) to the excel sheet

[alexcarol]

UI code: https://github.com/adobe/helix-importer-ui/blob/main/js/import/import.ui.js#L307

E.g.

return [{
      element: main,
      path: getPath(params),
      report: {
        icons: 'icons',
      },
    }];