adding an env: space onslaught

[thealokverse]

adding space_onslaught, a small C ocean env inspired by classic Space Invaders game.
i hope it will be helpful :)

the environment includes:

• 5x11 alien formation
• player movement and firing
• one player projectile
• up to three alien projectiles
• formation speed-up as aliens are destroyed
• three player lives
• score-based rewards
• terminal conditions for invasion or loss of all lives

verified locally with:

./build.sh space_onslaught --local