add IamReadyToPlayCommand & refactoring

src/main/kotlin/eventDemo/app/entity/GameId.kt

@@ -0,0 +1,15 @@
+package eventDemo.app.entity
+
+import eventDemo.libs.event.AggregateId
+import eventDemo.shared.GameIdSerializer
+import kotlinx.serialization.Serializable
+import java.util.UUID
+
+/**
+ * An [AggregateId] for a game.
+ */
+@JvmInline
+@Serializable(with = GameIdSerializer::class)
+value class GameId(
+    override val id: UUID = UUID.randomUUID(),
+) : AggregateId