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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,31 @@
+#pragma once
+#include <boost/asio/ip/tcp.hpp>
+#include <boost/system/error_code.hpp>
+#include <kj/async-io.h>
+
+namespace vanetza
+{
+namespace rpc
+{
+
+class AsioStream : public kj::AsyncIoStream
+{
+public:
+    AsioStream(boost::asio::ip::tcp::socket socket);
+
+    void shutdownWrite() override;
+    kj::Promise<void> write(const void* buffer, size_t size) override;
+    kj::Promise<void> write(kj::ArrayPtr<const kj::ArrayPtr<const kj::byte>> pieces) override;
+    kj::Promise<void> whenWriteDisconnected() override;
+    kj::Promise<size_t> tryRead(void* buffer, size_t minBytes, size_t maxBytes) override;
+
+private:
+    void signalDisconnect(const boost::system::error_code& ec);
+
+    boost::asio::ip::tcp::socket socket_;
+    kj::Maybe<kj::ForkedPromise<void>> disconnect_promise_;
+    kj::Maybe<kj::Own<kj::PromiseFulfiller<void>>> disconnect_fulfiller_;
+};
+
+} // namepsace rpc
+} // namespace vanetza