Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/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