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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,30 @@
+#include "byte_buffer_source.hpp"
+#include <algorithm>
+
+namespace vanetza
+{
+
+byte_buffer_source::byte_buffer_source(const ByteBuffer& buffer) :
+    m_begin(buffer.cbegin()), m_end(buffer.cend()) {}
+
+byte_buffer_source::byte_buffer_source(range r) :
+    m_begin(r.begin()), m_end(r.end()) {}
+
+byte_buffer_source::byte_buffer_source(iterator begin, iterator end) :
+    m_begin(begin), m_end(end) {}
+
+std::streamsize byte_buffer_source::read(char_type* buf, std::streamsize n)
+{
+    if (m_begin == m_end) {
+        return -1;
+    } else {
+        std::streamsize remaining = std::distance(m_begin, m_end);
+        std::streamsize consume = std::min(remaining, n);
+        std::copy(m_begin, m_begin + consume, buf);
+        std::advance(m_begin, consume);
+        return consume;
+    }
+}
+
+} // namespace vanetza
+