Issue #3031415 by markcarver: Overhaul CDN Providers API

bootstrap.drush.inc

deprecated.php

docs/Maintainers.md

docs/Theme-Settings.md

docs/plugins/Provider.md

docs/theme-settings.twig

js/theme-settings.js

scripts/bootstrap.php

scripts/gen-theme-setting-docs.php

src/Bootstrap.php

src/DeprecatedInterface.php

src/JsonResponse.php

src/Plugin/Alter/LibraryInfo.php

src/Plugin/Form/SystemThemeSettings.php

src/Plugin/Preprocess/ContainerHelpBlock.php

src/Plugin/Provider/ApiProviderBase.php

src/Plugin/Provider/BootstrapCdn.php

src/Plugin/Provider/Broken.php

src/Plugin/Provider/CdnAsset.php

src/Plugin/Provider/CdnAssets.php