[Rust] GrpoRewards: Implement Shaping Functions

[kyjohnso]

@nikg4 @taenin what other functions should we put into the GrpoRewards struct? other rewards functions? Here is a first cut from sonnet

3. Shaping Functions:
   • potential_based_shaping(state, next_state, gamma) - Implement potential-based reward shaping
   • curiosity_reward(state, next_state, prediction) - Generate intrinsic motivation rewards
   • constraint_penalty(state, action) - Apply penalties for constraint violations

Implement potential_based_shaping, curiosity_reward, constraint_penalty functions

Originally posted by @kyjohnso in #13