Add Flappy environment

[michal-kus]

After opening this PR, I noticed there's an existing one, but it appears inactive. The concern raised there has been addressed here.

Observation space:

• bird y position
• bird vertical velocity
• distance to the next pipe
• next pipe gap center y
• distance to the following pipe
• following pipe gap center y

Rewards:

• alive_reward = 0.01f,
• pass_reward = 1.0f,
• crash_reward = -1.0f,
• center_reward = 0.03f,