feat: add simulations

[dongsam]

Description

closes: #8

Bullet Points

• [ ]

References

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• Appropriate labels applied
• Targeted PR against correct branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #31 (3e37904) into master (5c9631b) will increase coverage by 18.17%.
The diff coverage is 85.00%.

❗ Current head 3e37904 differs from pull request most recent head dc1de84. Consider uploading reports for the commit dc1de84 to get more accurate results

@@             Coverage Diff             @@
##           master      #31       +/-   ##
===========================================
+ Coverage   58.29%   76.47%   +18.17%     
===========================================
  Files          11       12        +1     
  Lines         223      289       +66     
===========================================
+ Hits          130      221       +91     
+ Misses         71       53       -18     
+ Partials       22       15        -7     

Impacted Files	Coverage Δ
x/budget/simulation/operations.go	0.00% <0.00%> (ø)
x/budget/types/codec.go	0.00% <ø> (ø)
x/budget/types/keys.go	0.00% <0.00%> (ø)
x/budget/keeper/genesis.go	87.50% <88.88%> (+34.16%)	⬆️
x/budget/keeper/budget.go	79.74% <93.33%> (+8.31%)	⬆️
x/budget/keeper/grpc_query.go	100.00% <100.00%> (+100.00%)	⬆️
x/budget/simulation/decoder.go	100.00% <100.00%> (+80.00%)	⬆️
x/budget/types/genesis.go	100.00% <100.00%> (+18.18%)	⬆️
x/budget/types/params.go	95.55% <0.00%> (+8.88%)	⬆️
... and 2 more

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