[feature request] expects the El table tree to support custom expansion and lazy loading

### Existing Component
是

### Component Name
el-table

### Description
目前el-table支持自定义调用toggleRowExpansion方法自定义展开expand或者表格树，但是不支持懒加载形式，期望能优化支持

/store/watcher.js
```
toggleRowExpansionAdapter(row, expanded) {
const hasExpandColumn = this.states.columns.some(({ type }=> type === 'expand'));
if(hasExpandColumn) {
this.toggleRowExpansion(row, expanded);
} else {
this.toggleTreeExpansion(row,expanded); // 此处最好能支持调用到 this.loadOrToggle(row)
}
}
```

谢谢!