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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,50 @@
+#ifndef VARIANT_PDU_HPP_PHYRXMBW
+#define VARIANT_PDU_HPP_PHYRXMBW
+
+#include <vanetza/geonet/basic_header.hpp>
+#include <vanetza/geonet/common_header.hpp>
+#include <vanetza/geonet/header_variant.hpp>
+#include <vanetza/geonet/pdu.hpp>
+#include <vanetza/security/secured_message.hpp>
+#include <boost/optional/optional.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+class VariantPdu : public Pdu
+{
+public:
+    using SecuredMessage = security::SecuredMessage;
+
+    VariantPdu() = default;
+    VariantPdu(const VariantPdu&);
+    VariantPdu& operator=(const VariantPdu&);
+    VariantPdu(const Pdu&);
+    VariantPdu& operator=(const Pdu&);
+
+    BasicHeader& basic() override;
+    const BasicHeader& basic() const override;
+    CommonHeader& common() override;
+    const CommonHeader& common() const override;
+    HeaderVariant& extended_variant();
+    HeaderConstRefVariant extended_variant() const override;
+    SecuredMessage* secured() override;
+    const SecuredMessage* secured() const override;
+    void secured(const SecuredMessage&) override;
+    void secured(SecuredMessage&&);
+    std::unique_ptr<Pdu> clone() const override;
+
+private:
+    BasicHeader m_basic;
+    CommonHeader m_common;
+    HeaderVariant m_extended;
+    boost::optional<SecuredMessage> m_secured;
+};
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* VARIANT_PDU_HPP_PHYRXMBW */
+