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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,63 @@
+#include <vanetza/security/v2/payload.hpp>
+#include <vanetza/security/v2/length_coding.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+PayloadType get_type(const Payload& payload)
+{
+    return payload.type;
+}
+
+size_t get_size(const Payload& payload)
+{
+    size_t length = sizeof(PayloadType);
+    const size_t data = size(payload.data, OsiLayer::Network, max_osi_layer());
+    length += data;
+    length += length_coding_size(data);
+    return length;
+}
+
+size_t get_size(const ByteBuffer& buf)
+{
+    size_t size = buf.size();
+    size += length_coding_size(size);
+    return size;
+}
+
+void serialize(OutputArchive& ar, const Payload& payload)
+{
+    serialize(ar, payload.type);
+    serialize_length(ar, size(payload.data, OsiLayer::Network, max_osi_layer()));
+    serialize(ar, payload.data);
+}
+
+size_t deserialize(InputArchive& ar, Payload& payload)
+{
+    size_t size = sizeof(PayloadType);
+    PayloadType type;
+    deserialize(ar, type);
+    payload.type = type;
+
+    static const std::size_t data_length_limit = 4096;
+    const auto data_length = deserialize_length(ar);
+    if (data_length <= data_length_limit) {
+        size += length_coding_size(data_length);
+        size += data_length;
+        ByteBuffer buf(data_length);
+        ar.load_binary(buf.data(), buf.size());
+        payload.data = CohesivePacket(std::move(buf), OsiLayer::Network);
+    } else {
+        ar.fail(InputArchive::ErrorCode::ExcessiveLength);
+    }
+
+    return size;
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza