🤔合并单元格是否可以支持设置居中方式？

### 🖋 Description
使用 mergedCellsInfo 设置合并单元格信息后，展示出来的 meta 信息是否可以配置居中方式？
### 😊 Expected Behavior
可以在合并后的 **整个大单元格** 内，配置成垂直水平居中
### 😅 Current Behavior
默认是配置了 showText 所属单元格的居中方式

Start by reproducing the mergedCellsInfo example and inspect how the showText cell's alignment is applied to merged-cell metadata. The work is done when horizontal and vertical centering can be configured for the entire merged cell rather than only the cell containing showText.

Written by the indexing model from the issue text.