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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,52 @@
+#ifndef GBC_GAC_HEADER_HPP_FS5DH20M
+#define GBC_GAC_HEADER_HPP_FS5DH20M
+
+#include <vanetza/geonet/areas.hpp>
+#include <vanetza/geonet/position_vector.hpp>
+#include <vanetza/geonet/sequence_number.hpp>
+#include <vanetza/geonet/serialization.hpp>
+#include <vanetza/geonet/units.hpp>
+#include <cstddef>
+
+namespace vanetza
+{
+namespace geonet
+{
+namespace detail
+{
+
+/**
+ * GeoBroadcast GUC and GeoAnycast GAC have identical header layout.
+ * We use this class as common base class to reduce duplicate code.
+ */
+struct GbcGacHeader
+{
+    static constexpr std::size_t length_bytes = 20 + LongPositionVector::length_bytes;
+
+    SequenceNumber sequence_number;
+    uint16_t reserved1 = 0;
+    LongPositionVector source_position;
+    geo_angle_i32t geo_area_pos_latitude;
+    geo_angle_i32t geo_area_pos_longitude;
+    distance_u16t distance_a;
+    distance_u16t distance_b;
+    angle_u16t angle;
+    uint16_t reserved2 = 0;
+
+    void destination(const Area&);
+    GeodeticPosition position() const;
+    void position(const GeodeticPosition&);
+
+protected:
+    Area destination(const decltype(Area::shape)&) const;
+};
+
+void serialize(const GbcGacHeader&, OutputArchive&);
+void deserialize(GbcGacHeader&, InputArchive&);
+
+} // namespace detail
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* GBC_GAC_HEADER_HPP_FS5DH20M */
+