fix: fixing authz messages decoding issue in antehandlers

[spoo-bar]

Closes: #417

• Fixing the incorrect authz msg decoding in the x/rewards ante handlers.
• Using the nice helper func provided by x/authz to unwrap the msg instead of trying it explicitly

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.01 ⚠️

Comparison is base (be633fd) 69.39% compared to head (530a0c2) 69.39%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #418      +/-   ##
==========================================
- Coverage   69.39%   69.39%   -0.01%     
==========================================
  Files          67       67              
  Lines        3977     3976       -1     
==========================================
- Hits         2760     2759       -1     
  Misses       1122     1122              
  Partials       95       95              

Impacted Files	Coverage Δ
x/rewards/ante/ante_utils.go	68.75% <50.00%> (-0.95%)	⬇️

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[aelesbao]

Do we have any test covering this bug to avoid regressions?