Table header item used within models-table/table-header.

Each grouped-header should represents one item from ModelsTable.groupedHeaders.

Usage example:

{{#models-table data=data columns=columns as |mt|}}
  {{#mt.table as |table|}}
    {{#table.header as |header|}}
      {{#each header.groupedHeaders as |groupedHeader|}}
        {{header.grouped-header groupedHeader=groupedHeader}}
      {{/each}}
      {{! ... }}
    {{/table.header}}
    {{! ... }}
  {{/mt.table}}
  {{! .... }}
{{/models-table}}