feat: enable to add weekends in DatetimeEncoder

[MarieSacksick]

Description

Addresses skrub-data#710

I went for an attribute add_is_weekend which is a list[int].
Another option is possible. It would be to accept a boolean instead, and use the config to change what is considered a weekend and a weekday. I like it less for 2 reasons:

• more difficult to change
• and if the user has to deal with several datasets within the same session with different weekends, it's more painful to change config than a parameter (all the more in the same pipeline).

My option may seem weird and unintuitive to people who don't know that not the whole world have the same weekends (which is quite normal since almost everyone have the same weekends, cf wikipedia). I don't mind too much switching to a boolean.

None of these options are ok if, in the same dataset, there are several countries with different weekends, but, well, shouldn't be most cases, and this is intended to be "just" a helper, not to cover every possible option, isn't it?

Checklist

• I have read the contributing guidelines
• I have added tests that verify the bug fix
• I have added an entry to CHANGES.rst describing the fix
• My code follows the code style of this project
• I have checked my code and corrected any misspellings

How Has This Been Tested?

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