Issue #3444822 by Christian.wiedemann: [2.0.0-alpha3] Add allow_variant_expose source plugin

src/ComponentPluginManager.php

src/Element/ComponentElementBuilder.php

src/Element/ComponentForm.php

src/Element/ComponentPropsForm.php

src/Plugin/UiPatterns/PropType/VariantPropType.php

src/Plugin/UiPatterns/Source/SelectWidget.php

...dules/ui_patterns_test/components/card/card.component.yml

tests/modules/ui_patterns_test/components/card/card.twig