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FLASHING.md says how to take a newer version of the colleague's tree (copy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/security/v2/serialization.cpp

@@ -0,0 +1,65 @@
+#include <vanetza/security/v2/length_coding.hpp>
+#include <vanetza/security/v2/serialization.hpp>
+#include <cassert>
+#include <limits>
+#include <stdexcept>
+#include <type_traits>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+template<typename T>
+std::size_t trim_size_impl(T in, typename std::enable_if<std::is_same<T, std::size_t>::value>::type* = nullptr)
+{
+    return in;
+}
+
+template<typename T>
+std::size_t trim_size_impl(T in, typename std::enable_if<!std::is_same<T, std::size_t>::value>::type* = nullptr)
+{
+    if (in > std::numeric_limits<std::size_t>::max()) {
+        throw std::overflow_error("given size exceeds limits of std::size_t");
+    }
+    return static_cast<std::size_t>(in);
+}
+
+std::size_t trim_size(std::uintmax_t in)
+{
+    return trim_size_impl(in);
+}
+
+
+void serialize_length(OutputArchive& ar, std::uintmax_t length)
+{
+    ByteBuffer buf;
+    buf = encode_length(length);
+    for (auto it = buf.begin(); it != buf.end(); it++) {
+        ar << *it;
+    }
+}
+
+std::uintmax_t deserialize_length(InputArchive& ar)
+{
+    ByteBuffer buf(1);
+    ar >> buf[0];
+    const size_t leading = count_leading_ones(buf[0]);
+    buf.resize(leading + 1);
+    for (size_t c = 1; c <= leading; ++c) {
+        ar >> buf[c];
+    }
+    auto tup = decode_length(buf);
+    if (std::get<0>(tup) != buf.begin()) {
+        return std::get<1>(tup);
+    } else {
+        ar.fail(InputArchive::ErrorCode::ConstraintViolation);
+        return 0;
+    }
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza