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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,41 @@
+#ifndef TRANSMISSION_HPP_SDC4RMQE
+#define TRANSMISSION_HPP_SDC4RMQE
+
+#include <vanetza/access/data_rates.hpp>
+#include <vanetza/common/clock.hpp>
+#include <vanetza/dcc/profile.hpp>
+#include <cstddef>
+
+namespace vanetza
+{
+namespace dcc
+{
+
+class Transmission
+{
+public:
+    virtual Profile profile() const = 0;
+    virtual const access::DataRateG5* data_rate() const = 0;
+    virtual std::size_t body_length() const = 0;
+    virtual Clock::duration channel_occupancy() const;
+    virtual ~Transmission() = default;
+};
+
+struct TransmissionLite : public Transmission
+{
+    constexpr TransmissionLite(Profile dp, std::size_t len) : m_profile(dp), m_length(len) {}
+
+    Profile m_profile;
+    std::size_t m_length = 0; /*< length in bytes of MAC frame body */
+    const access::DataRateG5* m_data_rate = nullptr;
+
+    Profile profile() const override { return m_profile; }
+    const access::DataRateG5* data_rate() const override { return m_data_rate; }
+    std::size_t body_length() const override { return m_length; }
+};
+
+} // namespace dcc
+} // namespace vanetza
+
+#endif /* TRANSMISSION_HPP_SDC4RMQE */
+