feat: support labelRender prop

[inottn]

Allows custom rendering of the currently selected label, which can be used for value backfill but the corresponding option is missing and does not want to directly render the value.

Summary by CodeRabbit

• 新功能

  • 现在支持通过 labelRender 属性自定义 TreeSelect 组件已选项的标签显示内容。

• 测试

  • 新增了针对 labelRender 属性的用例，确保自定义标签渲染功能的正确性。