PieLabel 样式自定义——label居中

- [x] I have searched the [issues](https://github.com/antvis/f2/issues) of this repository and believe that this is not a duplicate.

### What problem does this feature solve?
业务上对于饼图有特殊的 UI 定制，需要将 label 文本居中在 PieLabel 指示线末端，试了一下调整单个 label1 / label2 的 offsetY，指示线会和文本重叠并且覆盖在文本上方。请问对于这样的 UI 场景，有什么调整的方法吗？

期望结果：

### What does the proposed API look like?
like...
```javascript
chart.PieLabel({
style: 'center'
})
```
?

No source file or test is named. Start by locating the PieLabel implementation and its label1/label2 offset handling, then compare the requested centered-at-line-end appearance with the linked image. Done means the API and resulting label behavior are defined clearly enough to validate.

Written by the indexing model from the issue text.