feat(`participation`): decay support

[lumtis]

Add in the params of the module decay information.

If enabled, a decay factor is applied to received rewards depending on the current timestamp

Decay factor change is linear from start to end. At end airdrop is no longer claimable.

[codecov[bot]]

Codecov Report

Merging #915 (a0a8a78) into develop (9366849) will increase coverage by 0.04%. The diff coverage is 22.09%.

@@             Coverage Diff             @@
##           develop     #915      +/-   ##
===========================================
+ Coverage    10.55%   10.60%   +0.04%     
===========================================
  Files          327      328       +1     
  Lines        75202    75537     +335     
===========================================
+ Hits          7939     8007      +68     
- Misses       67073    67340     +267     
  Partials       190      190              

Impacted Files	Coverage Δ
x/claim/types/params.pb.go	0.90% <0.37%> (-0.80%)	:arrow_down:
x/claim/types/params.go	60.86% <80.00%> (+14.71%)	:arrow_up:
x/claim/keeper/mission.go	96.51% <100.00%> (+0.30%)	:arrow_up:
x/claim/keeper/params.go	100.00% <100.00%> (ø)
x/claim/types/decay.go	100.00% <100.00%> (ø)

[lumtis]

Let's wait for https://github.com/tendermint/spn/pull/914 before merging this PR

Otherwise, I still open it for review @aljo242