Use the strategy pattern for reward shaping in `TemporalGoal` class.

[marfvr]

Is your feature request related to a problem? Please describe.

The class TemporalGoal hard-codes the behaviour of reward shaping, that can be controlled by the flag reward_shaping passed in the constructor.

Describe the solution you'd like

Make the approach more modular and customizable by introducing the RewardShaper class, such that it makes it easier for a developer to change the default behaviour.

Describe alternatives you've considered
None.

Additional context
None.