Decoder for numeric / decimal SQL Data type

[Gustl22]

Closes stablekernel/postgresql-dart#50,
closes stablekernel/postgresql-dart#108,
closes stablekernel/postgresql-dart#112,
closes stablekernel/postgresql-dart#162,
Contributes to stablekernel/postgresql-dart#164

Encoder may follows, if I've time.
At least I wanted to give a decoding option to the users who need it.

I decided to use String as exact representation of BigDecimal. An alternative would be to use decimal package , but I don't know if its a good idea, to use third party package as datatype. If wanted the users can use the package and convert the string themselves with the Decimal.parse method, or use double.parse(), if precision is not that important.

I think BigInt isn't an option, as need to save the fractional digits and therefore would again need a new wrapper class.

I also implemented a less precise numericToDouble method, but I think we should stick to the String representation:

/// Decode numeric / decimal to double which may has loss in precision.
double decodeNumericToDouble(List<int> bytes) {
  final uint8List = Uint8List.fromList(bytes);
  final buffer = ByteData.sublistView(uint8List);
  final nDigits = buffer.getInt16(0); // non-zero digits, data buffer length = 2 * nDigits
  var weight = buffer.getInt16(2); // weight of first digit
  final signByte = buffer.getInt16(4); // NUMERIC_POS, NEG, NAN, PINF, or NINF
  final dScale = buffer.getInt16(6); // display scale
  if (signByte == 0xc000) return double.nan;
  final sign = signByte == 0x4000 ? -1 : 1;
  final prependBytes = 8;
  var decimal = 0.0;
  for (var i = prependBytes; i < (nDigits * 2 + prependBytes); i += 2) {
    decimal += buffer.getUint16(i) * pow(10000, weight);
    weight--;
  }
  return decimal * sign;
}

[isoos]

I think this looks good, I have a small nit in the implementation. I don't think we need the lossy conversion to double, because the reason to use decimal/numeric is the need to preserve the precise number, without loosing precision.

Could you also make a note in the changelog about the new feature?

[Gustl22]

I made a note in the Changelog. If want to wait for the encoder, you can leave the PR open. Otherwise merging is fine, too :)

[isoos]

Thank you! Let me know if you want this to get released soon.

[Gustl22]

Thx! No hurry :) also can wait for the other PR

[MarkOSullivan94]

Thanks for adding support for numeric ❤️

What postgres release on pub will have this available in it?

Also off-topic but would it not be better having another repo for this package so people can report issues? Or is it recommended to report issues on the StableKernal repo @isoos?