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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,72 @@
+#include <vanetza/common/byte_view.hpp>
+#include <vanetza/common/byte_buffer_convertible.hpp>
+#include <cassert>
+#include <limits>
+
+namespace vanetza
+{
+
+namespace
+{
+
+boost::iterator_range<byte_view_iterator>
+make_safe_range(const ByteBuffer::const_iterator& begin, const ByteBuffer::const_iterator& end)
+{
+    if (begin < end) {
+        byte_view_iterator vbegin { begin };
+        byte_view_iterator vend { std::next(vbegin, std::distance(begin, end)) };
+        return boost::iterator_range<byte_view_iterator> { vbegin, vend };
+    } else {
+        byte_view_iterator empty;
+        return boost::iterator_range<byte_view_iterator> { empty, empty };
+    }
+}
+
+}
+
+byte_view_range::byte_view_range(const ByteBuffer::const_iterator& begin, const ByteBuffer::const_iterator& end) :
+    iterator_range(make_safe_range(begin, end))
+{
+}
+
+byte_view_range::byte_view_range(const byte_view_iterator& begin, const byte_view_iterator& end) :
+    iterator_range(begin, end)
+{
+}
+
+byte_view_range::byte_view_range(ByteBuffer&& _buffer) :
+    iterator_range(make_safe_range(_buffer.begin(), _buffer.end())), buffer(std::move(_buffer))
+{
+}
+
+ByteBuffer::const_pointer byte_view_range::data() const
+{
+    auto begin = this->begin();
+    return begin != this->end() ? begin.raw() : nullptr;
+}
+
+ByteBuffer::value_type byte_view_range::operator[](size_type pos) const
+{
+    static_assert(std::numeric_limits<size_type>::is_signed == false, "size_type shall be unsigned");
+    assert(pos < size());
+    return begin()[pos];
+}
+
+byte_view_range create_byte_view(ByteBuffer&& buffer)
+{
+    return byte_view_range { std::move(buffer) };
+}
+
+byte_view_range create_byte_view(const ByteBuffer& buffer)
+{
+    return byte_view_range { buffer.begin(), buffer.end() };
+}
+
+byte_view_range create_byte_view(const ByteBufferConvertible& convertible)
+{
+    ByteBuffer buffer;
+    convertible.convert(buffer);
+    return byte_view_range { std::move(buffer) };
+}
+
+} // namespace vanetza