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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,48 @@
+#include "hello_application.hpp"
+#include <chrono>
+#include <functional>
+#include <iostream>
+
+// This is a very simple application that sends BTP-B messages with the content 0xc0ffee.
+
+using namespace vanetza;
+
+HelloApplication::HelloApplication(boost::asio::io_context& io, std::chrono::milliseconds interval) :
+    timer_(io), interval_(interval)
+{
+    schedule_timer();
+}
+
+HelloApplication::PortType HelloApplication::port()
+{
+    return host_cast<uint16_t>(42);
+}
+
+void HelloApplication::indicate(const DataIndication&, UpPacketPtr)
+{
+    std::cout << "Hello application received a packet" << std::endl;
+}
+
+void HelloApplication::schedule_timer()
+{
+    timer_.expires_after(interval_);
+    timer_.async_wait(std::bind(&HelloApplication::on_timer, this, std::placeholders::_1));
+}
+
+void HelloApplication::on_timer(const boost::system::error_code& ec)
+{
+    if (ec != boost::asio::error::operation_aborted) {
+        DownPacketPtr packet { new DownPacket() };
+        packet->layer(OsiLayer::Application) = ByteBuffer { 0xC0, 0xFF, 0xEE };
+        DataRequest request;
+        request.transport_type = geonet::TransportType::SHB;
+        request.communication_profile = geonet::CommunicationProfile::ITS_G5;
+        request.its_aid = aid::CA;
+        auto confirm = Application::request(request, std::move(packet));
+        if (!confirm.accepted()) {
+            throw std::runtime_error("Hello application data request failed");
+        }
+
+        schedule_timer();
+    }
+}