feat(rewards): adding msgSrvr and querySrvr methods for contract premiums

[spoo-bar]

Closes: #255

• Added msgSrvr.SetFlatFee
• Added querySrvr.FlatFee
• Relevant tests

[codecov]

Codecov Report

Base: 69.43% // Head: 69.41% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (b7d7209) compared to base (6fe9369).
Patch coverage: 68.57% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #263      +/-   ##
==========================================
- Coverage   69.43%   69.41%   -0.02%     
==========================================
  Files          61       61              
  Lines        3510     3580      +70     
==========================================
+ Hits         2437     2485      +48     
- Misses       1001     1022      +21     
- Partials       72       73       +1     

Impacted Files	Coverage Δ
x/rewards/types/msg.go	41.12% <30.76%> (-2.75%)	⬇️
x/rewards/types/codec.go	52.94% <50.00%> (-0.40%)	⬇️
x/rewards/keeper/msg_server.go	96.00% <88.00%> (-4.00%)	⬇️
x/rewards/keeper/grpc_query.go	90.76% <100.00%> (+1.38%)	⬆️

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[fdymylja]

LGTM👻