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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,53 @@
+#include "byte_buffer_convertible.hpp"
+#include <algorithm>
+#include <iterator>
+
+namespace vanetza
+{
+namespace convertible
+{
+
+std::unique_ptr<byte_buffer> byte_buffer::duplicate() const
+{
+    ByteBuffer duplicate;
+    this->convert(duplicate);
+    std::unique_ptr<byte_buffer> result {
+        new byte_buffer_impl<ByteBuffer>(std::move(duplicate))
+    };
+    return result;
+}
+
+byte_buffer_impl<std::string>::byte_buffer_impl(const std::string& str) : m_buffer(str) {}
+byte_buffer_impl<std::string>::byte_buffer_impl(std::string&& str) : m_buffer(std::move(str)) {}
+
+void byte_buffer_impl<std::string>::convert(ByteBuffer& buffer) const
+{
+    buffer.clear();
+    std::copy(m_buffer.begin(), m_buffer.end(), std::back_inserter(buffer));
+}
+
+std::size_t byte_buffer_impl<std::string>::size() const
+{
+    return m_buffer.size();
+}
+
+std::unique_ptr<byte_buffer> byte_buffer_impl<std::nullptr_t>::duplicate() const
+{
+    return std::unique_ptr<byte_buffer> { new byte_buffer_impl<std::nullptr_t>() };
+}
+
+} // namespace convertible
+
+ByteBufferConvertible::ByteBufferConvertible(const ByteBufferConvertible& other) :
+    m_wrapper(other.m_wrapper->duplicate())
+{
+}
+
+ByteBufferConvertible& ByteBufferConvertible::operator=(const ByteBufferConvertible& other)
+{
+    m_wrapper = other.m_wrapper->duplicate();
+    return *this;
+}
+
+} // namespace vanetza
+