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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,130 @@
+#include <vanetza/security/secured_message.hpp>
+#include <boost/variant/apply_visitor.hpp>
+#include <boost/variant/static_visitor.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+
+SecuredMessageView::SecuredMessageView(const SecuredMessage& msg) :
+    m_variant(msg)
+{
+}
+
+struct ItsAidVisitor : boost::static_visitor<ItsAid>
+{
+    ItsAid operator()(const v2::SecuredMessage& msg) const
+    {
+        return get_its_aid(msg);
+    }
+
+    ItsAid operator()(const v3::SecuredMessage& msg) const
+    {
+        return msg.its_aid();
+    }
+};
+
+ItsAid get_its_aid(const SecuredMessage& msg)
+{
+    return boost::apply_visitor(ItsAidVisitor(), msg);
+}
+
+ItsAid get_its_aid(const SecuredMessageView& msg)
+{
+    return boost::apply_visitor(ItsAidVisitor(), msg);
+}
+
+std::size_t get_size(const SecuredMessage& msg)
+{
+    struct Visitor : boost::static_visitor<std::size_t>
+    {
+        std::size_t operator()(const v2::SecuredMessage& msg) const
+        {
+            return get_size(msg);
+        }
+
+        std::size_t operator()(const v3::SecuredMessage& msg) const
+        {
+            return msg.size();
+        }
+    };
+
+    return boost::apply_visitor(Visitor(), msg);
+}
+
+struct SerializeVisitor : boost::static_visitor<void>
+{
+    OutputArchive& m_archive;
+    SerializeVisitor(OutputArchive& ar) : m_archive(ar) {}
+
+    void operator()(const v2::SecuredMessage& msg)
+    {
+        serialize(m_archive, msg);
+    }
+
+    void operator()(const v3::SecuredMessage& msg)
+    {
+        serialize(m_archive, msg);
+    }
+};
+
+void serialize(OutputArchive& ar, const SecuredMessage& msg)
+{
+    SerializeVisitor visitor { ar };
+    boost::apply_visitor(visitor, msg);
+}
+
+void serialize(OutputArchive& ar, const SecuredMessageView& msg)
+{
+    SerializeVisitor visitor { ar };
+    boost::apply_visitor(visitor, msg);
+}
+
+std::size_t deserialize(InputArchive& ar, SecuredMessage& msg)
+{
+    struct Visitor : boost::static_visitor<std::size_t>
+    {
+        InputArchive& m_archive;
+        Visitor(InputArchive& ar) : m_archive(ar) {}
+
+        std::size_t operator()(v2::SecuredMessage& msg)
+        {
+            return deserialize(m_archive, msg);
+        }
+
+        std::size_t operator()(v3::SecuredMessage& msg)
+        {
+            return deserialize(m_archive, msg);
+        }
+    };
+
+    Visitor visitor(ar);
+    return boost::apply_visitor(visitor, msg);
+}
+
+struct PayloadCopyVisitor : boost::static_visitor<PacketVariant>
+{
+    PacketVariant operator()(const v2::SecuredMessage& msg) const
+    {
+        return msg.payload.data;
+    }
+
+    PacketVariant operator()(const v3::SecuredMessage& msg) const
+    {
+        return msg.payload();
+    }
+};
+
+PacketVariant get_payload_copy(const SecuredMessage& msg)
+{
+    return boost::apply_visitor(PayloadCopyVisitor(), msg);
+}
+
+PacketVariant get_payload_copy(const SecuredMessageView& msg)
+{
+    return boost::apply_visitor(PayloadCopyVisitor(), msg);
+}
+
+} // namespace security
+} // namespace vanetza