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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,58 @@
+#ifndef PDU_HPP_PQEC9PDO
+#define PDU_HPP_PQEC9PDO
+
+#include <vanetza/geonet/header_variant.hpp>
+#include <vanetza/geonet/serialization.hpp>
+#include <vanetza/security/secured_message.hpp>
+#include <cstddef>
+#include <memory>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+struct BasicHeader;
+struct CommonHeader;
+
+class Pdu;
+
+class ConstAccessiblePdu
+{
+public:
+    using SecuredMessage = security::SecuredMessage;
+
+    virtual const BasicHeader& basic() const = 0;
+    virtual const CommonHeader& common() const = 0;
+    virtual const SecuredMessage* secured() const = 0;
+    virtual HeaderConstRefVariant extended_variant() const = 0;
+    virtual std::unique_ptr<Pdu> clone() const = 0;
+    virtual ~ConstAccessiblePdu() = default;
+};
+
+class Pdu : public ConstAccessiblePdu
+{
+public:
+    using ConstAccessiblePdu::basic;
+    using ConstAccessiblePdu::common;
+    using ConstAccessiblePdu::secured;
+
+    virtual BasicHeader& basic() = 0;
+    virtual CommonHeader& common() = 0;
+    virtual SecuredMessage* secured() = 0;
+    virtual void secured(const SecuredMessage&) = 0;
+};
+
+void serialize(const ConstAccessiblePdu&, OutputArchive&);
+inline void serialize(const Pdu& pdu, OutputArchive& ar)
+{
+    serialize(static_cast<const ConstAccessiblePdu&>(pdu), ar);
+}
+
+std::size_t get_length(const ConstAccessiblePdu&);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* PDU_HPP_PQEC9PDO */
+