feat: add simple url encoder & decoder

[WZhuo]

The url encode and decode function will be used for generating partitions' path, so move it to the common util for later use.

[wgtmac]

Putting together all my comments, the code may look like this (disclaimer: gemini wrote it):

namespace {

bool IsUnreserved(unsigned char c) {
  return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') ||
         c == '-' || c == '.' || c == '_' || c == '~';
}

char ToHex(unsigned char v) {
  static constexpr std::array<char, 16> kHexChars = {
      '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
  return kHexChars[v & 0x0F];
}

int FromHex(char c) {
  if (c >= '0' && c <= '9') return c - '0';
  if (c >= 'a' && c <= 'f') return c - 'a' + 10;
  if (c >= 'A' && c <= 'F') return c - 'A' + 10;
  return -1;
}

}  // namespace

std::string UrlEncoder::Encode(std::string_view str_to_encode) {
  std::string escaped;
  escaped.reserve(str_to_encode.size() * 3 / 2);  // Heuristic reservation

  for (unsigned char c : str_to_encode) {
    if (IsUnreserved(c)) {
      escaped += static_cast<char>(c);
    } else {
      escaped += '%';
      escaped += ToHex(c >> 4);
      escaped += ToHex(c);
    }
  }
  return escaped;
}

std::string UrlEncoder::Decode(std::string_view str_to_decode) {
  std::string result;
  result.reserve(str_to_decode.size());

  for (size_t i = 0; i < str_to_decode.size(); ++i) {
    char c = str_to_decode[i];
    if (c == '%' && i + 2 < str_to_decode.size()) {
      int h1 = FromHex(str_to_decode[i + 1]);
      int h2 = FromHex(str_to_decode[i + 2]);

      if (h1 >= 0 && h2 >= 0) {
        result += static_cast<char>((h1 << 4) | h2);
        i += 2;
      } else {
        result += c;
      }
    } else if (c == '+') {
      result += ' ';
    } else {
      result += c;
    }
  }

  return result;
}

[WZhuo]

Rewrite the code translate from curl_easy_escape, curl_easy_unescape

[wgtmac]

Thanks @WZhuo for working on this and @HuaHuaY for the review!