Issue #3472936: Implement endpoint authentication

composer.json

config/install/rdf_sync.settings.yml

config/schema/rdf_sync.schema.yml

.../rdf_sync_published/tests/Kernel/RdfSyncPublishedTest.php

rdf_sync.install

rdf_sync.services.yml

src/Attribute/RdfSyncConnector.php

src/Form/SettingsForm.php

src/Model/VirtuosoEndpoint.php

src/Plugin/rdf_sync/Connector/Virtuoso.php

src/RdfSyncConnection.php

src/RdfSyncConnectionInterface.php

src/RdfSyncConnectorPluginInterface.php

src/RdfSyncConnectorPluginManager.php

src/RdfSyncConnectorPluginPluginBase.php

src/RdfSyncSynchronizer.php

src/RdfSyncTriples.php

src/RdfSyncTriplesInterface.php

...ules/rdf_sync_test/config/schema/rdf_sync_test.schema.yml

...ules/rdf_sync_test/src/Plugin/rdf_sync/Connector/ARC2.php