Update cms_plugins.py - now using cms.api to add columns

[aacimov]

Using cms.api to make the plugin compatible with django-cms v4 as well. This patch also works for django-cms v3.

Description

Changed cms_plugins.py save_model method to make the plugin work with django-cms v3 and v4 by using the cms.api for adding columns.

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
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