[FR] Support for database row's children

### Description

It is currently not possible to assign any child to a row, which makes it not possible to implement important project management features such as sub tasks/issues.

### Impact

Users who use AppFlowy mainly for project management.

### Additional Context

_No response_

No files, tests, or entry points are named in the issue. Start by locating the database row implementation and its existing relationship or nesting behavior, then determine the data model and UI scope needed for row children. Done should support assigning children to a row for use cases such as subtasks or sub-issues, with coverage for the resulting behavior.

Written by the indexing model from the issue text.