[Feature Request] I have a demand that el-table use treeData and lazy render to make cell merge

### Existing Component
是

### Component Name
el-table

### Description
By reading the underlying source code of ElementUi, I found that the private stores attribute inside the el-table component manages lazy loaded tree data, and there is no API exposed to call. When I use the arrayspanmethod attribute to merge cells, I can only modify the underlying lazytreenodemap and tabledata. There are great risks. I hope it can be supported