树形结构自定义显示展开/折叠icon

### 🏷 Version

| Package | Version |
| -------------- | ------- |
| @antv/s2 | 1.47.1 |
| @antv/s2-react | |
| @antv/s2-vue | |

### Sheet Type

- [x] PivotSheet
- [ ] TableSheet
- [ ] GridAnalysisSheet
- [ ] StrategySheet
- [ ] EditableSheet

### 🖋 Description
树形结构时，没有子集的节点也出现了展开/折叠的icon；在定义rows时确定了层级的字段，但每一层的层级其实不确定的，就会出现没有子集的节点也显示了icon；重新RowCell里面的showTreeIcon方法，从meta里面也拿不到相应的数据进行判断，每次展开/折叠后meta里面level和children的值是变化的，找不到一个合适可用的判断条件

### 🔗 Reproduce Link

https://codesandbox.io/embed/quiet-water-hxzmmj?fontsize=14&hidenavigation=1&theme=dark

### 😊 Expected Behavior
可以自行判断是否显示展开/折叠的icon

### 😅 Current Behavior

Start with RowCell and its showTreeIcon method, then reproduce the behavior using the linked CodeSandbox with @antv/s2 1.47.1. Trace the available metadata before and after expanding or collapsing rows; done means leaf nodes do not show the icon and icon visibility can be determined according to the configured tree data.

Written by the indexing model from the issue text.