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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,36 @@
+#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>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+void serialize(const ConstAccessiblePdu& pdu, OutputArchive& ar)
+{
+    serialize(pdu.basic(), ar);
+    if (pdu.secured()) {
+        security::serialize(ar, *pdu.secured());
+    } else {
+        geonet::serialize(pdu.common(), ar);
+        boost::serialize(pdu.extended_variant(), ar);
+    }
+}
+
+std::size_t get_length(const ConstAccessiblePdu& pdu)
+{
+    std::size_t length = BasicHeader::length_bytes;
+    if (pdu.secured()) {
+        length += security::get_size(*pdu.secured());
+    } else {
+        length += CommonHeader::length_bytes;
+        length += get_length(pdu.extended_variant());
+    }
+    return length;
+}
+
+} // namespace geonet
+} // namespace vanetza