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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,25 @@
+#include "data_rates.hpp"
+#include <cassert>
+
+namespace vanetza
+{
+namespace access
+{
+
+std::size_t DataRateG5::data_length(std::size_t psdu) const
+{
+    static const unsigned service_bits = 16;
+    static const unsigned tail_bits = 6;
+
+    const unsigned body = service_bits + tail_bits + psdu * 8;
+    unsigned padding = body % m_coded_bits_per_symbol;
+    if (padding > 0) {
+        padding = m_coded_bits_per_symbol - padding;
+    }
+
+    assert((body + padding) % 8 == 0);
+    return (body + padding) / 8;
+}
+
+} // namespace access
+} // namespace vanetza