chore: Format code with golangci-lint fmt

[alexandear]

• Change script/fmt.sh to use the golangci-lint fmt command. Internally, it uses gofmt, gci, gofumpt, goimports simultaneously.
• Set gci.sections, gofumpt.extra-rules in golangci-lint config and reformat examples.

Inspired by discussion comments in #3938.

[merchantmoh-debug]

@alexandear - Genius. Very good idea.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.45%. Comparing base (1c42933) to head (f7b6d82).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3949   +/-   ##
=======================================
  Coverage   92.45%   92.45%           
=======================================
  Files         203      203           
  Lines       14980    14980           
=======================================
  Hits        13850    13850           
  Misses        927      927           
  Partials      203      203           

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