fix(x/rewards): create block rewards for flat fees

[spoo-bar]

Ref: #336

• Modified MinFeeDecorator such that when a contract is found with flatfees configured, the relevant rewards records are created
• Modified DeductFeeDecorator such that when a contract is found with flatfees configured, the relevant amount is sent from the sender's acc to the rewards collector account
• Some refactor of x/rewards antehandlers in general with moving common stuff to ante_utils
• Added TestRewardsFlatFees to e2e rewards_tests which checks that post a successful tx completion, the relevant records for contract premium are created.

[codecov]

Codecov Report

Patch coverage: 56.92% and project coverage change: -0.32 ⚠️

Comparison is base (1bb75b4) 69.63% compared to head (a6cd05d) 69.32%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #338      +/-   ##
==========================================
- Coverage   69.63%   69.32%   -0.32%     
==========================================
  Files          63       64       +1     
  Lines        3777     3804      +27     
==========================================
+ Hits         2630     2637       +7     
- Misses       1070     1088      +18     
- Partials       77       79       +2     

Impacted Files	Coverage Δ
x/rewards/keeper/flat_fee.go	76.47% <0.00%> (-23.53%)	⬇️
x/rewards/ante/fee_deduction.go	56.81% <41.17%> (-4.23%)	⬇️
x/rewards/ante/ante_utils.go	69.69% <69.69%> (ø)
app/ante.go	58.82% <100.00%> (ø)
x/rewards/ante/min_cons_fee.go	78.57% <100.00%> (+0.95%)	⬆️

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