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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,53 @@
+#ifndef HEADER_HPP_FNKGEM7C
+#define HEADER_HPP_FNKGEM7C
+
+#include <vanetza/common/byte_order.hpp>
+#include <vanetza/common/serialization.hpp>
+#include <vanetza/net/packet_variant.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace btp
+{
+
+typedef uint16be_t port_type;
+
+// interactive packet transport
+struct HeaderA
+{
+    static constexpr std::size_t length_bytes = 4;
+
+    port_type destination_port;
+    port_type source_port;
+};
+
+static_assert(sizeof(HeaderA) == HeaderA::length_bytes, "Wrong size");
+
+void serialize(OutputArchive&, const HeaderA&);
+void deserialize(InputArchive&, HeaderA&);
+HeaderA parse_btp_a(ChunkPacket&);
+HeaderA parse_btp_a(CohesivePacket&);
+HeaderA parse_btp_a(PacketVariant&);
+
+// non-interactive packet transport
+struct HeaderB
+{
+    static constexpr std::size_t length_bytes = 4;
+
+    port_type destination_port;
+    uint16be_t destination_port_info;
+};
+
+static_assert(sizeof(HeaderB) == HeaderB::length_bytes, "Wrong size");
+
+void serialize(OutputArchive&, const HeaderB&);
+void deserialize(InputArchive&, HeaderB&);
+HeaderB parse_btp_b(ChunkPacket&);
+HeaderB parse_btp_b(CohesivePacket&);
+HeaderB parse_btp_b(PacketVariant&);
+
+} // namepsace btp
+} // namespace vanetza
+
+#endif /* HEADER_HPP_FNKGEM7C */