add explosive bullet power up

TanksServer/GameLogic/SpawnPowerUp.cs

@@ -0,0 +1,18 @@
+namespace TanksServer.GameLogic;
+
+internal sealed class SpawnPowerUp(
+    IOptions<GameRules> options,
+    MapEntityManager entityManager
+) : ITickStep
+{
+    private readonly double _spawnChance = options.Value.PowerUpSpawnChance;
+
+    public Task TickAsync(TimeSpan delta)
+    {
+        if (Random.Shared.NextDouble() > _spawnChance * delta.TotalSeconds)
+            return Task.CompletedTask;
+
+        entityManager.SpawnPowerUp();
+        return Task.CompletedTask;
+    }
+}