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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,41 @@
+#pragma once
+
+#include "link_layer.hpp"
+#include <vanetza/access/interface.hpp>
+#include <vanetza/rpc/asio_event_loop.hpp>
+#include <vanetza/rpc/asio_event_port.hpp>
+#include <vanetza/rpc/asio_stream.hpp>
+#include <vanetza/rpc/link_layer_client.hpp>
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/ip/tcp.hpp>
+#include <boost/program_options/options_description.hpp>
+#include <kj/async.h>
+
+class RpcLinkLayer : public LinkLayer
+{
+public:
+    /**
+     * Create RPC link layer
+     * \param io ASIO context
+     * \param socket TCP socket connected to RPC server
+     */
+    RpcLinkLayer(boost::asio::io_context& io, boost::asio::ip::tcp::socket socket);
+    ~RpcLinkLayer() noexcept;
+
+    void request(const vanetza::access::DataRequest&, std::unique_ptr<vanetza::ChunkPacket>) override;
+    void indicate(IndicationCallback) override;
+    void set_source_address(const vanetza::MacAddress&) override;
+
+    void radio_technology(const std::string&);
+    void enable_debug(bool);
+    static void add_options(boost::program_options::options_description&);
+
+private:
+    boost::asio::io_context& io_;
+    vanetza::rpc::AsioEventPort event_port_;
+    vanetza::rpc::AsioEventLoop event_loop_;
+    vanetza::rpc::AsioStream asio_stream_;
+    kj::WaitScope wait_scope_;
+    vanetza::rpc::LinkLayerClient client_;
+    IndicationCallback callback_;
+};