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/geonet/header_variant.hpp

@@ -0,0 +1,48 @@
+#ifndef HEADER_VARIANT_HPP
+#define HEADER_VARIANT_HPP
+
+#include <vanetza/geonet/serialization.hpp>
+#include <vanetza/geonet/beacon_header.hpp>
+#include <vanetza/geonet/gbc_header.hpp>
+#include <vanetza/geonet/shb_header.hpp>
+#include <vanetza/geonet/tsb_header.hpp>
+#include <boost/variant/variant.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+typedef boost::variant<BeaconHeader, GeoBroadcastHeader, ShbHeader, TsbHeader> HeaderVariant;
+typedef boost::variant<BeaconHeader&, GeoBroadcastHeader&, ShbHeader&, TsbHeader&> HeaderRefVariant;
+typedef boost::variant<const BeaconHeader&, const GeoBroadcastHeader&, const ShbHeader&, const TsbHeader&> HeaderConstRefVariant;
+
+/** \brief get the length of the underlying header type
+ *
+ * \param header the header to get the length from
+ * \return std::size_t
+ *
+ */
+std::size_t get_length(const HeaderVariant& header);
+std::size_t get_length(const HeaderRefVariant& header);
+std::size_t get_length(const HeaderConstRefVariant& header);
+
+} // namespace geonet
+} // namespace vanetza
+
+namespace boost
+{
+
+/** \brief serialize the underlying header type to an OutputArchive
+ *
+ * \param header the header to serialize
+ * \param ar the archive to write to
+ *
+ */
+void serialize(const vanetza::geonet::HeaderVariant& header, vanetza::OutputArchive& ar);
+void serialize(const vanetza::geonet::HeaderRefVariant& header, vanetza::OutputArchive& ar);
+void serialize(const vanetza::geonet::HeaderConstRefVariant& header, vanetza::OutputArchive& ar);
+
+} // namespace boost
+
+#endif // HEADER_VARIANT_HPP