message: Add GetBufferSpan() function to bufferbuilder
  This will return the finished internal buffer data as a span
diff: @@ -1239,6 +1239,14 @@ class FlatBufferBuilder {
  return buf_.data();
  }
  + /// @brief Get the serialized buffer (after you call `Finish()`) as a span.
  + /// @return Returns a constructed flatbuffers::span that is a view over the
  + /// FlatBuffer data inside the buffer.
  + flatbuffers::span<uint8_t> GetBufferSpan() const {
  + Finished();
  + return flatbuffers::span<uint8_t>(buf_.data(), buf_.size());
  + }
  +
  /// @brief Get a pointer to an unfinished buffer.
  /// @return Returns a `uint8_t` pointer to the unfinished buffer.
  uint8_t *GetCurrentBufferPointer() const { return buf_.data(); }
  