Add support for getvalidators

[mwherman2000]

https://github.com/neo-project/neo/blob/b9bb68b94e9231f9533ebbed2ca98ffbda0b938a/neo/Network/RPC/RpcServer.cs#L200:

                case "getvalidators":
                    {
                        var validators = Blockchain.Default.GetValidators();
                        return Blockchain.Default.GetEnrollments().Select(p =>
                        {
                            JObject validator = new JObject();
                            validator["publickey"] = p.PublicKey.ToString();
                            validator["votes"] = p.Votes.ToString();
                            validator["active"] = validators.Contains(p.PublicKey);
                            return validator;
                        }).ToArray();
                    }