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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,59 @@
+#ifndef BASIC_HEADER_HPP_8QS7WLG3
+#define BASIC_HEADER_HPP_8QS7WLG3
+
+#include <vanetza/common/bit_number.hpp>
+#include <vanetza/geonet/lifetime.hpp>
+#include <vanetza/geonet/mib.hpp>
+#include <vanetza/geonet/serialization.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+struct DataRequest;
+struct ShbDataRequest;
+
+/// NextHeaderBasic specified in ETSI EN 302 636-4-1 v1.2.1, section 8.6.3
+enum class NextHeaderBasic : uint8_t
+{
+    Any = 0,
+    Common = 1,
+    Secured = 2,
+};
+
+/// BasicHeader specified in ETSI EN 302 636-4-1 v1.2.1, section 8.6
+struct BasicHeader
+{
+    BasicHeader();
+    BasicHeader(const MIB&);
+    BasicHeader(const DataRequest&, const MIB&);
+    BasicHeader(const ShbDataRequest&, const MIB&);
+
+    static constexpr std::size_t length_bytes = 3 + sizeof(Lifetime);
+
+    BitNumber<unsigned, 4> version;
+    NextHeaderBasic next_header; // 4 bit
+    uint8_t reserved;
+    Lifetime lifetime;
+    uint8_t hop_limit;
+};
+
+/**
+ * \brief Serializes a BasicHeader into a binary archive
+ * \param basic to serialize
+ * \param ar to serialize in
+ */
+void serialize(const BasicHeader&, OutputArchive&);
+
+/**
+ * \brief Deserializes a BasicHeader from a binary archive
+ * \param basic to deserialize
+ * \param ar with a serialized BasicHeader at the beginning
+ */
+void deserialize(BasicHeader&, InputArchive&);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* BASIC_HEADER_HPP_8QS7WLG3 */