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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,29 @@
+#include <gtest/gtest.h>
+#include <vanetza/common/byte_buffer_source.hpp>
+#include <boost/iostreams/stream_buffer.hpp>
+#include <istream>
+
+using namespace vanetza;
+
+TEST(ByteBufferSource, read) {
+    const ByteBuffer buf = { 0x01, 0x11, 0x12, 0x22, 0x23, 0x33, 0x34, 0x44 };
+    byte_buffer_source source(buf);
+    boost::iostreams::stream_buffer<byte_buffer_source> stream(source);
+
+    std::istream is(&stream);
+    ASSERT_TRUE(is.good());
+
+    char read = '\0';
+    unsigned read_bytes = 0;
+    for (uint8_t byte : buf) {
+        is >> read;
+        EXPECT_NE(byte, '\0');
+        EXPECT_EQ(byte, read);
+        ++read_bytes;
+    }
+    EXPECT_EQ(read_bytes, 8);
+
+    is >> read;
+    EXPECT_TRUE(is.eof());
+}
+