Add AddAmount/SubAmount methods to sdk.Coin

[hallazzang]

Description

closes: #9079

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• Targeted PR against correct branch (see CONTRIBUTING.md)
• 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 tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[hallazzang]

Need to decide whether to add AddAmountInt64/SubAmountInt64 methods to follow NewInt64Coin convention.

[codecov]

Codecov Report

Merging #9091 (8eb9758) into master (a4c7fd7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #9091   +/-   ##
=======================================
  Coverage   58.82%   58.83%           
=======================================
  Files         580      580           
  Lines       32643    32648    +5     
=======================================
+ Hits        19203    19208    +5     
  Misses      11174    11174           
  Partials     2266     2266           

Impacted Files	Coverage Δ
types/coin.go	93.27% <100.00%> (+0.15%)	⬆️

[robert-zaremba]

utACK