Add `arrow_metadata(<expr>)` UDF

[adriangb]

As discussed in #19335 it would be nice to have a function arrow_metadata similar to arrow_typeof that returns the metadata from it's input field as a Map<String, String> column. Ideally it could support two signatures arrow_metadata(column_or_expr) -> Map<String, String> and arrow_metadata(column_or_expr, 'key') -> String.