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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,66 @@
+#ifndef PAYLOAD_HPP_R8IXQBSL
+#define PAYLOAD_HPP_R8IXQBSL
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/net/packet.hpp>
+#include <vanetza/security/v2/serialization.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+/// PayloadType specified in TS 103 097 v1.2.1, section 5.3
+enum class PayloadType : uint8_t
+{
+    Unsecured = 0,
+    Signed = 1,
+    Encrypted = 2,
+    Signed_External = 3,
+    Signed_And_Encrypted = 4,
+};
+
+/// Payload specified in TS 103 097 v1.2.1, section 5.2
+struct Payload
+{
+    PayloadType type;
+    PacketVariant data;
+};
+
+/**
+ * \brief Determines PayloadType to a given Payload
+ * \param payload
+ * \return type
+ */
+PayloadType get_type(const Payload&);
+
+/**
+ * \brief Calculates size of Payload
+ * \param payload
+ * \return number of octets needed to serialize the Payload
+ */
+size_t get_size(const Payload&);
+
+/**
+ * \brief Serializes Payload into a binary archive
+ * \param ar to serialize in
+ * \param payload to serialize
+ */
+void serialize(OutputArchive& ar, const Payload&);
+
+/**
+ * \brief Deserializes Payload from a binary archive
+ * \param ar with serialized Payload at the beginning
+ * \param payload to deserialize
+ * \return size of the deserialized payload
+ */
+size_t deserialize(InputArchive& ar, Payload&);
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza
+
+#endif /* PAYLOAD_HPP_R8IXQBSL */