Add add-on (DLC) support (#1519)

Add add-on content support

Co-authored-by: nmzik <Nmzik@mail.ru>

src/common/additionalContent.h

@@ -0,0 +1,23 @@
+#ifndef KYTY_COMMON_ADDITIONAL_CONTENT_H_
+#define KYTY_COMMON_ADDITIONAL_CONTENT_H_
+
+#include <filesystem>
+#include <string>
+#include <vector>
+
+namespace Common {
+
+struct AddContentEntry {
+	std::string           content_id;
+	std::string           entitlement_label;
+	std::filesystem::path mount_dir;
+	std::string           title;
+};
+
+// Finds packages for this title, deduplicated by entitlement label.
+std::vector<AddContentEntry> FindAddContent(const std::filesystem::path& dir,
+                                            const std::string&           title_id);
+
+} // namespace Common
+
+#endif /* KYTY_COMMON_ADDITIONAL_CONTENT_H_ */