FEAT - Extend the `has_nulls` selector so it can take a fraction of null values

[rcap107]

At the moment, the has_nulls selector selects only columns that have at least one null value. It would be useful to add a threshold (similar to that used in DropUninformative) to select all columns that have a number of null values larger than the given threshold.

This issue is related to #1723

[jeromedockes]

yeah maybe that's a good alternative to #1723 and #1714 , letting users easily select such columns and do what encoding they prefer with them