Add Backup DNS Seed for Dogecoin

[UsaRandom]

Using backup dns seed, seed2.multidoge.org

see: dogecoinfoundation/libdogecoin#239

Was unable to connect to the dogecoin network using just seed.multidoge.org. This update fixes it.

Test example:

using NBitcoin;
using NBitcoin.Protocol;
using System;
using System.Linq;
using System.Threading;

public class Program
{
    public static void Main(string[] args)
    {
        // Define the Dogecoin mainnet network
        Network dogecoinMain = NBitcoin.Altcoins.Dogecoin.Instance.Mainnet;

        // Configure node connection settings
        var nodeSettings = new NodeConnectionParameters
        {
            UserAgent = "/nbitcoin-dogeseedtest:1.0.0/",
            Services = NodeServices.Network
        };

        // Start from the genesis block
        uint256 lastHash = dogecoinMain.GenesisHash;
        int height = 0;

        // Use NodesGroup for robust peer management
        using (var nodes = new NodesGroup(dogecoinMain, nodeSettings))
        {
            nodes.Connect(); // Connect to the network
            Console.WriteLine("Connected to Dogecoin network.");

            while (true)
            {
                try
                {
                    // Find a connected node
                    Node node = nodes.ConnectedNodes.FirstOrDefault();
                    if (node == null)
                    {
                        Console.WriteLine("No connected nodes. Waiting...");
                        Thread.Sleep(5000);
                        continue;
                    }

                    // Request block inventory
                    var getBlocks = new GetBlocksPayload
                    {
                        BlockLocators = new BlockLocator { Blocks = new List<uint256> { lastHash } }
                    };
                    node.SendMessage(getBlocks);

                    // Receive inventory with timeout
                    InvPayload inv = node.ReceiveMessage<InvPayload>(TimeSpan.FromSeconds(30));
                    if (inv.Inventory.Count == 0 || inv.Inventory[0].Type != InventoryType.MSG_BLOCK)
                    {
                        Console.WriteLine("No more blocks to process.");
                        break;
                    }

                    // Request the next block
                    var nextHash = inv.Inventory[0].Hash;
                    var getData = new GetDataPayload();
                    getData.Inventory.Add(new InventoryVector(InventoryType.MSG_BLOCK, nextHash));
                    node.SendMessage(getData);

                    // Receive the block
                    BlockPayload blockMessage = node.ReceiveMessage<BlockPayload>(TimeSpan.FromSeconds(30));
                    Block block = blockMessage.Object;

                    // Verify block sequence
                    if (block.Header.HashPrevBlock == lastHash)
                    {
                        height++;
                        lastHash = block.GetHash();
                        Console.WriteLine($"Block height: {height}");
                    }
                    else
                    {
                        Console.WriteLine($"Block out of order: {block.GetHash()}");
                    }
                }
                catch (Exception ex)
                {
                    Console.WriteLine($"Error: {ex.Message}. Retrying in 5 seconds...");
                    Thread.Sleep(5000);
                }
            }
        }
    }
}