Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/vanetza/security/v3/persistence.cpp

@@ -0,0 +1,36 @@
+#include <vanetza/common/serialization.hpp>
+#include <vanetza/security/v3/persistence.hpp>
+#include <fstream>
+#include <iterator>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v3
+{
+
+Certificate load_certificate_from_file(const std::string& certificate_path)
+{
+    Certificate certificate;
+
+    std::ifstream certificate_src;
+    certificate_src.open(certificate_path, std::ios::in | std::ios::binary);
+    vanetza::ByteBuffer buffer(std::istreambuf_iterator<char>(certificate_src), {});
+    certificate.decode(buffer);
+
+    return certificate;
+}
+
+void save_certificate_to_file(const std::string& certificate_path, const Certificate& certificate)
+{
+    std::ofstream dest;
+    dest.open(certificate_path.c_str(), std::ios::out | std::ios::binary);
+
+    OutputArchive archive(dest);
+    serialize(archive, certificate);
+}
+
+} // namespace v3
+} // namespace security
+} // namespace vanetza