[FEA] Implement Java/JNI function applyNullMask

[ttnghia]

We need a function in Java/JNI to apply a null mask onto some provided input column. This functionality is used a lot in Spark-rapids plugin.

Currently, we can achieve this using copy_if_else but we have to create and provide a null scalar for the else (if not valid) option, which is a bit inconvenient and incurs additional maintenance overhead.

[bdice]

@ttnghia Can you explain your concerns about maintenance? It seems like copy_if_else is a good way to do this.

Otherwise, can you use cudf::column::set_null_mask directly?

[ttnghia]

Yes, copy_if_else is the current workaround for doing it:

std::unique_ptr<column> copy_if_else(
  scalar const& lhs,
  column_view const& rhs,
  column_view const& boolean_mask,

However, we always have to create the null scalar (lhs) and destroy it after using. That is the overhead that I mentioned.

For implementing this feature, we will need superimpose_nulls instead of cudf::column::set_null_mask since we are dealing with generic (nested) columns.