feat(reward): emit events

[aljo242]

Closes #732

Emits events for Create, Update, Remove actions on the RewardPool type. Additionally emits EventRewardsDistributed during k.DistributeRewards().

[codecov]

Codecov Report

Merging #752 (8ce640b) into develop (045c15b) will decrease coverage by 0.07%.
The diff coverage is 3.57%.

@@             Coverage Diff             @@
##           develop     #752      +/-   ##
===========================================
- Coverage    10.99%   10.92%   -0.08%     
===========================================
  Files          284      285       +1     
  Lines        66878    67488     +610     
===========================================
+ Hits          7356     7372      +16     
- Misses       59347    59940     +593     
- Partials       175      176       +1     

Impacted Files	Coverage Δ
x/reward/types/events.pb.go	0.83% <0.83%> (ø)
x/reward/keeper/reward_distribution.go	80.57% <62.50%> (-1.11%)	⬇️
x/reward/keeper/msg_set_reward.go	93.02% <100.00%> (+0.52%)	⬆️