Draft: feat: [Lightgallery] Add admin settings form (refs #3537187)

• Add LightgallerySettingsTrait to provide access to settings
• Add SettingsForm to manage Lightgallery settings
• Update EntityReferenceLightgalleryFormatterTrait to use settings
• Update LightgalleryThumbnailFormatterTrait to use settings
• Add schema for lightgallery.settings.yml
• Update lightgallery.settings.yml to include new settings
• Add help text for each setting in the schema
• Ensure settings are available in the formatter traits

Closes #3537225