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FLASHING.md says how to take a newer version of the colleague's tree (copy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.cpp

@@ -0,0 +1,81 @@
+#include <vanetza/geonet/header_variant.hpp>
+
+namespace boost
+{
+
+using namespace vanetza::geonet;
+
+class HeaderVariantVisitor : public boost::static_visitor<>
+{
+public:
+    HeaderVariantVisitor(OutputArchive& ar) :
+        m_archive(ar)
+    {
+    }
+
+    template<typename T>
+    void operator()(const T& header)
+    {
+        serialize(header, m_archive);
+    }
+
+private:
+    OutputArchive& m_archive;
+};
+
+void serialize(const HeaderVariant& header, OutputArchive& ar)
+{
+    HeaderVariantVisitor visit(ar);
+    boost::apply_visitor(visit, header);
+}
+
+void serialize(const HeaderRefVariant& header, OutputArchive& ar)
+{
+    HeaderVariantVisitor visitor(ar);
+    boost::apply_visitor(visitor, header);
+}
+
+void serialize(const HeaderConstRefVariant& header, OutputArchive& ar)
+{
+    HeaderVariantVisitor visitor(ar);
+    boost::apply_visitor(visitor, header);
+}
+
+} // namespace boost
+
+
+namespace vanetza
+{
+namespace geonet
+{
+
+class HeaderVariantLengthVisitor : public boost::static_visitor<std::size_t>
+{
+public:
+    template<typename T>
+    std::size_t operator()(const T&)
+    {
+        return T::length_bytes;
+    }
+};
+
+std::size_t get_length(const HeaderVariant& header)
+{
+    HeaderVariantLengthVisitor visit;
+    return boost::apply_visitor(visit, header);
+}
+
+std::size_t get_length(const HeaderRefVariant& header)
+{
+    HeaderVariantLengthVisitor visitor;
+    return boost::apply_visitor(visitor, header);
+}
+
+std::size_t get_length(const HeaderConstRefVariant& header)
+{
+    HeaderVariantLengthVisitor visitor;
+    return boost::apply_visitor(visitor, header);
+}
+
+} // namespace geonet
+} // namespace vanetza