add custom navigation group to role resource

[OthmanHaba]

No description provided.

[what-the-diff]

PR Summary

• Introduction of new configuration option
  A new configuration option, custom_navigation_group, has been added to the configuration file config/filament-shield.php. This allows for more customization of the user interface.

• Update in 'getNavigationGroup' method
  The method getNavigationGroup in the file RoleResource.php now uses the new method getCustomNavigationGroup from the utility file to retrieve the value of custom_navigation_group from the configuration. Therefore, it enhances the way the system retrieves and utilizes this configuration option.

• New method in Utils
  A new method named getCustomNavigationGroup is introduced in Utils.php. Its purpose is to fetch the value of the newly added custom_navigation_group from the configuration. This small function is responsible for a significant part of this update.