Add missing energy reward field

[Calm0]

Used if a reward is defined when spending an energy

[CLAassistant]

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[zyro]

@Calm0 this should be covered by the available_rewards field, shouldn't it? https://github.com/heroiclabs/hiro/pull/73/files#diff-f80cd1900cdad452f1365c009b40be6eccdc69f4756b4d1cf2d2ae49654fd547R1661

[Calm0]

@zyro Initially I had the idea of wanting the rolled reward to be part of the Energy so it can be accessed from anywhere as the "last rolled reward" but now I don't think that it makes much sense to keep a record of that especially that the reward will only be rolled when the client requests that by spending it so the EnergySpendReward response should be enough at the spot to do whatever with it.
Closing this 👍