chore: parse includes and assetlists while parsing zone definition

src/ObjCommon/SearchPath/SearchPathMultiInputStream.cpp

@@ -0,0 +1,15 @@
+#include "SearchPathMultiInputStream.h"
+
+SearchPathMultiInputStream::SearchPathMultiInputStream(ISearchPath& searchPath)
+    : m_search_path(searchPath)
+{
+}
+
+std::unique_ptr<std::istream> SearchPathMultiInputStream::OpenIncludedFile(const std::string& filename, const std::string& sourceFile)
+{
+    auto foundFileToInclude = m_search_path.Open(filename);
+    if (!foundFileToInclude.IsOpen() || !foundFileToInclude.m_stream)
+        return nullptr;
+
+    return std::move(foundFileToInclude.m_stream);
+}