[FEA] Add `sliced_child_begin()` and `sliced_child_end()` iterators for `column_view`

Currently, the `column_view` class offers the accessor `child_begin()` and `child_end()` iterators that allow iterating over the children of a column. However, when we want to access the sliced children column (in many places), we have to do that manually. It would be great if we can cache the sliced children and provide iterators `sliced_child_begin()` and `sliced_child_end()` that allow quickly iterating over them.

Since different classes (`structs_column_view` and `lists_column_view`) have different interfaces for generating `column_view` of sliced child, we may have to implement the proposed iterators separately in these classes instead of implementing in the `column_view` base class.