* Added required attribute to settings/plugins, refactor: allValues
- added 'required' attribute to InvenTreeBaseSetting
- added 'check_all_settings'
- added 'all_settings' to get a list of all defined settings
- refactored 'allValues' to use new 'all_settings' function
- added docs for new 'check_setting' function on plugin SettingsMixin
* Fix typing to be compatible with python 3.9
* trigger: ci
* Fixed **kwargs bug and added tests
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