Implement maps_column_view abstraction over LIST<STRUCT<K,V>>

[mythrocks]

Fixes #9109.

This commit adds a map abstraction over a column_view of type LIST<STRUCT<K,V>>, where K and V are key and value types. A list column of structs with two members may thus be viewed as a map column.

maps_column_view is to a LIST<STRUCT<K,V>> column what lists_column_view is to a LIST column.

The maps_column_view abstraction provides methods to fetch lists of keys and values (as LIST<K> and LIST<V> respectively). It also provides map lookup methods to find the values corresponding to a specified key, for each row in the "map" column.

E.g.

auto input_column = get_list_of_structs_col();
// input_column == [ {1:10, 2:20}, {1:100, 3:300}, {2:2000, 3:3000, 4:4000} ];

auto maps_view = cudf::jni::maps_column_view{input_column->view()};
auto keys = maps_view.keys();     // keys   == [ {1,2},   {1,3},      {2,3,4} ];
auto values = maps_view.values(); // values == [ {10,20}, {100, 300}, {2000, 3000, 4000} ];

auto lookup_1 = maps_view.get_values_for( *make_numeric_scalar(1) );
// lookup_1 = [ {10, 100, null} ];

This abstraction should help replace the Java/JNI map_lookup and map_contains kernels, which only handles MAP<STRING, STRING>.

[mythrocks]

This PR is a work in progress, and in draft.

[jrhemstad]

Given that a map type doesn't exist in libcudf, I don't think this code should live in the libcudf header/source tree. I would expect this to live in the JNI layer or whatever we're calling the libcudf-spark companion library.

[mythrocks]

I would expect this to live in the JNI layer or whatever we're calling the libcudf-spark companion library.

I'll move this over to the java/src/native directory before moving out of draft.

[jlowe]

Other than the minor comment about min() vs. max(), lgtm from the Java perspective.

[ajschmidt8]

Approving ops-codeowner file changes

[mythrocks]

Thanks for the reviews, all.

I'll merge this once I have rerun tests.

[mythrocks]

@gpucibot merge

[mythrocks]

Thank you for the reviews, all. This change was just merged.