ENH: Use the cloud probability mask for S2-L2A data to retrieve clouds

[remi-braun]

Cloud probability masks seems to be better to retrieve the clouds from S2 L2A data.
It could be better to load the cloud mask from this file.

In the same effort, handling CLDPRB and SNWPRB masks could be a good effort and also making the method open_mask public.

This is true for S2 data, but also for any constellation that has masks. Private methods should already exist and maybe need to be rewritten.