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FLASHING.md says how to take a newer version of the colleague's tree (copy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/security/v2/trailer_field.cpp

@@ -0,0 +1,82 @@
+#include <vanetza/security/exception.hpp>
+#include <vanetza/security/v2/trailer_field.hpp>
+#include <boost/variant/get.hpp>
+#include <cassert>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+TrailerFieldType get_type(const TrailerField& field)
+{
+    struct trailerFieldVisitor : public boost::static_visitor<TrailerFieldType>
+    {
+        TrailerFieldType operator()(const Signature&)
+        {
+            return TrailerFieldType::Signature;
+        }
+    };
+    trailerFieldVisitor visit;
+    return boost::apply_visitor(visit, field);
+}
+
+size_t get_size(const TrailerField& field)
+{
+    size_t size = sizeof(TrailerFieldType);
+    struct trailerFieldVisitor : public boost::static_visitor<size_t>
+    {
+        size_t operator()(const Signature& sig)
+        {
+            return get_size(sig);
+        }
+    };
+    trailerFieldVisitor visit;
+    size += boost::apply_visitor(visit, field);
+    return size;
+}
+
+void serialize(OutputArchive& ar, const TrailerField& field)
+{
+    struct trailer_visitor : public boost::static_visitor<>
+    {
+        trailer_visitor(OutputArchive& ar) :
+            m_archive(ar)
+        {
+        }
+        void operator()(const Signature& sig)
+        {
+            serialize(m_archive, sig);
+        }
+        OutputArchive& m_archive;
+    };
+    TrailerFieldType type = get_type(field);
+    serialize(ar, type);
+    trailer_visitor visitor(ar);
+    boost::apply_visitor(visitor, field);
+}
+
+size_t deserialize(InputArchive& ar, TrailerField& field)
+{
+    size_t size = 0;
+    TrailerFieldType type;
+    deserialize(ar, type);
+    size += sizeof(TrailerFieldType);
+    switch (type) {
+        case TrailerFieldType::Signature: {
+            Signature sig;
+            size += deserialize(ar, sig);
+            field = sig;
+            break;
+        }
+        default:
+            throw deserialization_error("Unknown TrailerFieldType");
+    }
+    return size;
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza