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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,90 @@
+#pragma once
+#include <kj/async-io.h>
+#include <vanetza/dcc/channel_load.hpp>
+#include <vanetza/net/chunk_packet.hpp>
+#include <vanetza/net/cohesive_packet.hpp>
+#include <vanetza/net/mac_address.hpp>
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include <mutex>
+#include <string>
+
+namespace vanetza
+{
+
+namespace access { class DataRequest; }
+
+namespace rpc
+{
+
+class Logger;
+
+class LinkLayerClient
+{
+public:
+    enum class ErrorCode
+    {
+        Ok,
+        InvalidArgument,
+        Unsupported,
+        InternalError,
+    };
+
+    enum class Technology
+    {
+        Unspecified,
+        ITS_G5,
+        LTE_V2X,
+    };
+
+    struct Indication
+    {
+        Indication(vanetza::ByteBuffer);
+
+        vanetza::MacAddress source;
+        vanetza::MacAddress destination;
+        vanetza::CohesivePacket packet;
+        Technology technology = Technology::Unspecified;
+    };
+
+    struct Identity
+    {
+        std::uint64_t id = 0;
+        std::uint32_t version = 0;
+        std::string info;
+    };
+
+    using IndicationCallback = std::function<void(Indication)>;
+    using ChannelLoadReportCallback = std::function<void(dcc::ChannelLoad)>;
+
+    LinkLayerClient(kj::Timer&, kj::AsyncIoStream&, Logger* = nullptr);
+    ~LinkLayerClient();
+
+    kj::Promise<Identity> identify();
+    void request(const access::DataRequest&, std::unique_ptr<ChunkPacket>);
+    void indicate(IndicationCallback callback);
+    void report_channel_load(ChannelLoadReportCallback callback);
+    kj::Promise<ErrorCode> set_source_address(const MacAddress&);
+
+    void configure(Technology);
+    void add_task(kj::Promise<void>&&);
+
+private:
+    class Context;
+
+    void do_indicate(Indication);
+    void do_report(dcc::ChannelLoad);
+
+    std::unique_ptr<Context> context_;
+    std::mutex callback_mutex_;
+    IndicationCallback indication_callback_;
+    ChannelLoadReportCallback cbr_callback_;
+    Technology technology_ = Technology::Unspecified;
+};
+
+const char* stringify(LinkLayerClient::ErrorCode);
+const char* stringify(LinkLayerClient::Technology);
+
+} // namespace rpc
+} // namespace vanetza