Issue #2020001 by linclark, jesse.d: Make datatype callback handling more flexible.

core/modules/forum/config/rdf.mapping.node.forum.yml

core/modules/rdf/lib/Drupal/rdf/SchemaOrgDataConverter.php

...odules/rdf/lib/Drupal/rdf/Tests/CommentAttributesTest.php

core/modules/rdf/lib/Drupal/rdf/Tests/CrudTest.php

core/modules/rdf/lib/Drupal/rdf/Tests/NodeAttributesTest.php

...es/rdf/lib/Drupal/rdf/Tests/RdfMappingUpgradePathTest.php

core/modules/rdf/lib/Drupal/rdf/Tests/RdfaAttributesTest.php

.../modules/rdf/lib/Drupal/rdf/Tests/StandardProfileTest.php

...odules/rdf/lib/Drupal/rdf/Tests/TrackerAttributesTest.php

core/modules/rdf/rdf.install

core/modules/rdf/rdf.module

...ndard/config/rdf.mapping.comment.comment_node_article.yml

...standard/config/rdf.mapping.comment.comment_node_page.yml

core/profiles/standard/config/rdf.mapping.node.article.yml

core/profiles/standard/config/rdf.mapping.node.page.yml