Add HKDF (SHA256)-Expand

[awaik]

HKDF is a key derivation function that has two parts: first "extract" and second "expand". For some encryption tasks, we need only the expand part. I made a code for that for my project and, I guess, it would be great to add to the lib.

  Future<SecretKey> expandKey({
    required SecretKey secretKey,
    List<int> info = const <int>[],
  }) async {
    // T(0)
    var bytes = const <int>[];

    // T(1), T(2), ...
    final hashLength = hmac.hashAlgorithm.hashLengthInBytes;
    final n = outputLength ~/ hashLength;
    final result = Uint8List(outputLength);
    for (var i = 0; i <= n; i++) {
      final sink = await hmac.newMacSink(secretKey: secretKey);
      sink.add(bytes);
      if (info.isNotEmpty) {
        sink.add(info);
      }
      final added = <int>[0xFF & (1 + i)];
      sink.add(added);
      sink.close();
      final mac = await sink.mac();
      bytes = mac.bytes;
      final offset = i * hashLength;
      if (offset + bytes.length <= result.length) {
        result.setAll(offset, bytes);
      } else {
        result.setAll(offset, bytes.take(result.length - offset));
      }
    }
    return SecretKey(result);
  }