Function random_string

examples/server/httplib.h:4728–4739

https://stackoverflow.com/questions/440133/how-do-i-create-a-random-alpha-numeric-string-in-c/440240#answer-440240

// https://stackoverflow.com/questions/440133/how-do-i-create-a-random-alpha-numeric-string-in-c/440240#answer-440240
inline std::string random_string(size_t length) {
  auto randchar = []() -> char {
    const char charset[] = "0123456789"
                           "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                           "abcdefghijklmnopqrstuvwxyz";
    const size_t max_index = (sizeof(charset) - 1);
    return charset[static_cast<size_t>(std::rand()) % max_index];
  };
  std::string str(length, 0);
  std::generate_n(str.begin(), length, randchar);
  return str;
}

class ContentProviderAdapter {
public: