implement federated group shares

apps/cloud_federation_api/lib/Config.php

...ederation_api/lib/Controller/RequestHandlerController.php

apps/federatedfilesharing/lib/AppInfo/Application.php

apps/federatedfilesharing/lib/FederatedShareProvider.php

apps/federatedfilesharing/lib/Notifications.php

...ratedfilesharing/lib/ocm/CloudFederationProviderFiles.php

apps/files_sharing/appinfo/database.xml

apps/files_sharing/appinfo/info.xml

apps/files_sharing/lib/Controller/ShareAPIController.php

apps/files_sharing/lib/External/Manager.php

lib/private/Share/Constants.php

lib/private/Share20/Manager.php

lib/private/Share20/ProviderFactory.php

lib/public/Share/IManager.php