[Feature Request] Are there some way to customize summary row or display html

### Existing Component
是

### Component Name
Table

### Description
hi~ I want to show total with **fixed** row at the bottom of table, and it seems the only way is summary row.
But I also want to **customize** content with html, for example, i want to display two-line content in the cell.

Is possible that support summary slot or richtext in summary row?

你好，我想问一下，element现在有方法可以实现table组件最后一行固定的同时，支持自定义cell的内容吗（比如支持多行数据）?

如果可以，希望可以考虑一下支持summary/footer slot，或者可以在summary/footer行支持富文本 ~