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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,63 @@
+#ifndef SECURED_PDU_HPP_TVERYI91
+#define SECURED_PDU_HPP_TVERYI91
+
+#include <vanetza/common/byte_buffer_convertible.hpp>
+#include <vanetza/geonet/common_header.hpp>
+#include <vanetza/geonet/extended_pdu.hpp>
+#include <vanetza/geonet/header_variant.hpp>
+#include <vanetza/geonet/serialization.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+/**
+ * Secured PDU consists of GeoNetworking headers belonging to a secured message's payload
+ */
+class SecuredPdu
+{
+public:
+    template<typename EXT>
+    SecuredPdu(const ExtendedPdu<EXT>& pdu) :
+        common(pdu.common()), extended(pdu.extended())
+    {
+    }
+
+    SecuredPdu(const Pdu&);
+
+    CommonHeader common;
+    HeaderVariant extended;
+};
+
+void serialize(const SecuredPdu&, OutputArchive&);
+
+/**
+ * Cast secured PDU from byte buffer convertible
+ * \param conv source convertible
+ * \return secured PDU pointer or nullptr if cast failed
+ */
+SecuredPdu* secured_pdu_cast(ByteBufferConvertible&);
+
+} // namespace geonet
+
+namespace convertible
+{
+
+template<>
+struct byte_buffer_impl<geonet::SecuredPdu> : public byte_buffer
+{
+    byte_buffer_impl(const geonet::SecuredPdu&);
+    void convert(ByteBuffer&) const override;
+    std::size_t size() const override;
+    std::unique_ptr<byte_buffer> duplicate() const override;
+
+    geonet::SecuredPdu pdu;
+};
+
+} // namespace convertible
+
+} // namespace vanetza
+
+#endif /* SECURED_PDU_HPP_TVERYI91 */
+