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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,35 @@
+#ifndef UDP_LINK_HPP_A16QFBX3
+#define UDP_LINK_HPP_A16QFBX3
+
+#include "link_layer.hpp"
+#include <vanetza/common/byte_buffer.hpp>
+#include <boost/asio/ip/udp.hpp>
+#include <array>
+
+// forward declaration
+class EthernetDevice;
+
+class UdpLink : public LinkLayer
+{
+public:
+    UdpLink(boost::asio::io_context&, const boost::asio::ip::udp::endpoint&, const EthernetDevice&);
+
+    void indicate(IndicationCallback) override;
+    void request(const vanetza::access::DataRequest&, std::unique_ptr<vanetza::ChunkPacket>) override;
+
+private:
+    void do_receive();
+
+    static constexpr std::size_t layers_ = num_osi_layers(vanetza::OsiLayer::Link, vanetza::OsiLayer::Application);
+
+    boost::asio::ip::udp::endpoint multicast_endpoint_;
+    boost::asio::ip::udp::socket tx_socket_;
+    boost::asio::ip::udp::socket rx_socket_;
+    std::array<vanetza::ByteBuffer, layers_> tx_buffers_;
+    vanetza::ByteBuffer rx_buffer_;
+    boost::asio::ip::udp::endpoint rx_endpoint_;
+    IndicationCallback callback_;
+};
+
+#endif /* UDP_LINK_HPP_A16QFBX3 */
+