feat(campaign): add createdAt field

[lumtis]

Add a createdAt field into the campaign object. Set at campaign creation

[codecov]

Codecov Report

Merging #796 (e5ce80d) into develop (cd0cfc7) will increase coverage by 0.01%.
The diff coverage is 20.83%.

❗ Current head e5ce80d differs from pull request most recent head 96a01f9. Consider uploading reports for the commit 96a01f9 to get more accurate results

@@             Coverage Diff             @@
##           develop     #796      +/-   ##
===========================================
+ Coverage    10.81%   10.82%   +0.01%     
===========================================
  Files          286      286              
  Lines        68267    68305      +38     
===========================================
+ Hits          7384     7397      +13     
- Misses       60707    60734      +27     
+ Partials       176      174       -2     

Impacted Files	Coverage Δ
x/campaign/types/campaign.pb.go	0.56% <0.00%> (-0.04%)	⬇️
x/campaign/keeper/msg_create_campaign.go	86.66% <100.00%> (+2.45%)	⬆️
x/campaign/types/campaign.go	100.00% <100.00%> (ø)
x/campaign/simulation/store.go	84.45% <0.00%> (+3.37%)	⬆️