* Plugin reload mechanism
- Wrap reload_plugins with mutex lock
- Add methods for calculating plugin registry hash
* Perform plugin reload at critical entry points to the registry
- Background worker will correctly reload registry before performing tasks
- Ensures that the background worker plugin regsistry is up to date
* 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
* Added typing for settings
* Refactored common.BaseInvenTreeSetting model to make it more generic
* Use older syntax for union types
* Added protected option to typing
* Remove now unused code
* Remove old 'get_kwargs' method as it is replaced by 'get_filters_for_instance'
* Trigger ci
