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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,54 @@
+#include <vanetza/geonet/secured_pdu.hpp>
+#include <vanetza/geonet/serialization_buffer.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+SecuredPdu::SecuredPdu(const Pdu& pdu) :
+    common(pdu.common()), extended(pdu.extended_variant())
+{
+}
+
+void serialize(const SecuredPdu& pdu, OutputArchive& ar)
+{
+    serialize(pdu.common, ar);
+    serialize(pdu.extended, ar);
+}
+
+SecuredPdu* secured_pdu_cast(ByteBufferConvertible& convertible)
+{
+    using convertible_pdu_t = convertible::byte_buffer_impl<SecuredPdu>;
+    auto convertible_pdu = dynamic_cast<convertible_pdu_t*>(convertible.ptr());
+    return convertible_pdu ? &convertible_pdu->pdu : nullptr;
+}
+
+} // namespace geonet
+
+namespace convertible
+{
+
+byte_buffer_impl<geonet::SecuredPdu>::byte_buffer_impl(const geonet::SecuredPdu& _pdu) :
+    pdu(_pdu)
+{
+}
+
+void byte_buffer_impl<geonet::SecuredPdu>::convert(ByteBuffer& buffer) const
+{
+    geonet::serialize_into_buffer(pdu, buffer);
+}
+
+std::size_t byte_buffer_impl<geonet::SecuredPdu>::size() const
+{
+    using namespace geonet;
+    return CommonHeader::length_bytes + get_length(pdu.extended);
+}
+
+std::unique_ptr<byte_buffer> byte_buffer_impl<geonet::SecuredPdu>::duplicate() const
+{
+    return std::unique_ptr<byte_buffer_impl> { new byte_buffer_impl(*this) };
+}
+
+} // namespace convertible
+} // namespace vanetza