deterministicGenerateK function
Implementation
ECSignature deterministicGenerateK(Uint8List hash, Uint8List x) {
final signer = ECDSASigner(null, HMac(SHA256Digest(), 64));
var pkp = PrivateKeyParameter(ECPrivateKey(_decodeBigInt(x), secp256k1));
signer.init(true, pkp);
return signer.generateSignature(hash) as ECSignature;
}