crud中的children嵌套功能建议

children[嵌套](https://aisuda.bce.baidu.com/amis/zh-CN/components/crud#%E5%B5%8C%E5%A5%97)功能：目前 children 是一个数组，如果 children 是一个字典，在其中可以放一个 items 数组（数据）和一个 columns 数组（动态列）就好了。这样children就不必与parent的列一模一样，并且可以在children的每行配置操作按钮，这些操作按钮与parent的也不一样。

Start with the CRUD nested-children documentation linked in the issue and inspect the existing children configuration shape. Determine how an object containing items and columns would fit the current API, including distinct per-row actions for nested rows. Done means nested children can use their own data, dynamic columns, and row actions without matching the parent configuration.

Written by the indexing model from the issue text.