[C++][Python] How to get subset of values from struct arrays and list arrays

* use struct_field kernel
* select a subset of struct fields by casting the array to array with struct subset
* for array of lists you can choose an element by index with list_element kernel
* UDF or custom compute kernel

Start by reviewing the struct_field and list_element kernels named in the issue, along with the existing C++ and Python cookbook organization. Determine where examples for struct-field subsets, list-element selection, casting, and UDF or custom compute kernels belong. Done means the cookbook clearly demonstrates these operations for both languages.

Written by the indexing model from the issue text.