Use simdjson::pad to pad strings to be parsed by simdjson

See joyeecheung#8 for a reference.

This might also be applicable to

size_t json_length = package_config.raw_json.size();
package_config.raw_json.append(simdjson::SIMDJSON_PADDING, ' ');
simdjson::padded_string_view json_view(package_config.raw_json.data(),
json_length,
package_config.raw_json.size());

There is another place where we are manually padding the buffer, but I am not so sure whether using simdjson::pad is better than the current handling. I'd suggest first time contributors to avoid it unless you can assess the impact between the current approach and using simdjson::pad and find the latter to be preferrable.

if (!StringViewToUTF8(message,
&message_utf8,
&message_utf8_length,
simdjson::SIMDJSON_PADDING)) {