feat: Provide possibility to display specific countries in SelectCountryBone

[cheeaza]

There was no possibility to limit the set of available countries in a selectcountryBone.
I added the parameter countries to limit it. I also updated the docstring.

[phorward]

Hello @cheeaza,

thanks for creating this pull request. Your implementation might work, but I think the SelectCountryBone is implemented very bad and old fashioned, and should generally be refactored so that it gets back closer to the SelectBone.

1. The current globals ISO2CODES and ISO3CODES should become part of the SelectCountryBone class, so they can be accessed via SelectCountryBone.ISO2 and SelectCountryBone.ISO3.
2. The signature of the __init__ function should become

   def __init__(
       self,
       *,
       descr="Country",  # default description
       values=SelectCountryBone.ISO2,  # default code set
       **kwargs
    ):

3. Afterwards, the behavior defaults to the same as before. In case one wants to filter only specific countries, this can be done with a simple comprehension, like

   deliver_to = SelectCountryBone(
       values={
           k: v for k, v in SelectCountryBone.ISO2 if k in ("de", "en")
       }
   )

4. The current codes-parameter could still be supported with a deprecation warning.

I hope you're with me in this case. Would you like to propose an alternative to this pull request?

[phorward]

This is not consistent with the signature of the function below!