Add `NewColumn()` to `ColumnArray`

[theory]

Returns a new column of the proper type for inserting new values. Useful for contexts in which the item type isn't immediately known. Not a blocker for my project but would be a much nicer API to use the current approach, which is:

auto items = CreateColumnByType(
	col_type.As<clickhouse::ArrayType>()->GetItemType()->GetName()
);