Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/vanetza/security/v2/trailer_field.hpp

@@ -0,0 +1,69 @@
+#ifndef TRAILER_FIELD_HPP_3PDKGWCQ
+#define TRAILER_FIELD_HPP_3PDKGWCQ
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/security/v2/signature.hpp>
+#include <boost/variant/variant.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+/// TrailerFieldType specified in TS 103 097 v1.2.1, section 5.7
+enum class TrailerFieldType : uint8_t
+{
+    Signature = 1
+};
+
+/// TrailerField specified in TS 103 097 v1.2.1, section 5.6
+using TrailerField = boost::variant<Signature>;
+
+/**
+ * \brief Determines TrailerFieldType to a given TrailerField
+ * \param field
+ * \return type
+ */
+TrailerFieldType get_type(const TrailerField&);
+
+/**
+ * \brief Calculates size of a TrailerField
+ * \param field
+ * \return number of octets needed to serialize the TrailerField
+ */
+size_t get_size(const TrailerField&);
+
+/**
+ * \brief Serializes a TrailerField into a binary archive
+ * \param ar to serialize in
+ * \param field to serialize
+ */
+void serialize(OutputArchive&, const TrailerField&);
+
+/**
+ * \brief Deserializes a TrailerField from a binary archive
+ * \param ar with a serialized TrailerField at the beginning
+ * \param field to deserialize
+ * \return size of the deserialized TrailerField
+ */
+size_t deserialize(InputArchive&, TrailerField&);
+
+/**
+ * \brief resolve type for matching TrailerFieldType
+ *
+ * This is kind of the reverse function of get_type(const TrailerField&)
+ */
+template<TrailerFieldType>
+struct trailer_field_type;
+
+template<>
+struct trailer_field_type<TrailerFieldType::Signature> { using type = Signature; };
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza
+
+#endif /* TRAILER_FIELD_HPP_3PDKGWCQ */