Add CLI to specify dest for relay messages

The dest can be specified as a 4 char ASCII string, or as an IPv4 address.

src/Client.zig

@@ -39,10 +39,10 @@ fn broadcastSaprusMessage(msg: SaprusMessage, udp_port: u16, allocator: Allocato
     _ = try sock.sendTo(dest_addr, msg_bytes);
 }
 
-pub fn sendRelay(payload: []const u8, allocator: Allocator) !void {
+pub fn sendRelay(payload: []const u8, dest: [4]u8, allocator: Allocator) !void {
     const msg = SaprusMessage{
         .relay = .{
-            .header = .{ .dest = .{ 70, 70, 70, 70 } },
+            .header = .{ .dest = dest },
             .payload = payload,
         },
     };