Add controlled lists of recording types and external hosting options

src/MDExternalHostingSet.php

@@ -0,0 +1,20 @@
+<?PHP
+/**
+ * Contains a class for controlling the list of available external sources.
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Class containing available external sources.
+ */
+final class MDExternalHostingSet extends MDValueSet {
+
+    const DEFAULT_EXTERNAL_HOSTING = 'Archive.org';
+
+    const EXTERNAL_SOURCES = [
+        'Archive.org',
+    ];
+
+}