Implement SpawnManager for AI Enemies

[dgcnz]

Currently, only one enemy "spawns" at the start of the game. More enemies should spawn with time or the amount of enemies currently on the game.

[dgcnz]

Follow: https://codingchronicles.com/unity-vr-development/day-23-of-100-days-of-vr-creating-a-spawning-system-for-enemies-in-unity