Formalize the name of the nested Field in a list

[gruuya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
As discussed in apache/datafusion#13468 (comment), currently it is a convention to have list field member fields (i.e. the inner field) names be item. However this is not formalized in any way, and consequently there's a large number of item literals thrown around the codebase.

Describe the solution you'd like
Add a Field::LIST_FIELD_DEFAULT_NAME constant set to item, and use it in Field::new_list_field.

Also replace any calls to Field::new with explicit item name to use Field::new_list_field instead.

Describe alternatives you've considered
Could also make a free-standing as opposed to an associated constant (or not extract into a constant at all).

Additional context
Spin-off out of apache/datafusion#13468