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FLASHING.md says how to take a newer version of the colleague's tree (copy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