feat(x/rewards): Contract premium - emit events

[spoo-bar]

Closes #273

• Emit event when contract flat fee is set or updated

[codecov]

Codecov Report

Base: 69.49% // Head: 69.40% // Decreases project coverage by -0.10% ⚠️

Coverage data is based on head (9837dfc) compared to base (9fce480).
Patch coverage: 41.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
- Coverage   69.49%   69.40%   -0.10%     
==========================================
  Files          61       61              
  Lines        3590     3602      +12     
==========================================
+ Hits         2495     2500       +5     
- Misses       1022     1029       +7     
  Partials       73       73              

Impacted Files	Coverage Δ
x/rewards/types/events.go	0.00% <0.00%> (ø)
x/rewards/keeper/flat_fee.go	100.00% <100.00%> (ø)

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