Use linter goheader to ensure license header exists

[Warashi]

What this PR does:

• enable goheader within the golangci-lint's config
• fix the warnings

Why we need it:

To ensure the license header exists

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[Warashi]

I ran make lint/go and got the result below.

❯ make lint/go
docker run --rm -e GOCACHE=/repo/.cache/go-build -e GOLANGCI_LINT_CACHE=/repo/.cache/golangci-lint -v /home/warashi/ghq/github.com/pipe-cd/pipecd:/repo -w /repo -it golangci/golangci-lint@sha256:4e53bfe25ef2f1e14a95da42d694211080f40d118730541ce1513a83cf7587ec  golangci-lint run -v
INFO golangci-lint has version 1.62.2 built with go1.23.3 from 89476e7a on 2024-11-25T14:16:01Z 
INFO [config_reader] Config search paths: [./ /repo / /root] 
INFO [config_reader] Used config file .golangci.yml 
INFO [lintersdb] Active 12 linters: [depguard gocritic goheader goimports gosimple ineffassign misspell prealloc staticcheck stylecheck unconvert unparam] 
INFO [loader] Go packages loading at mode 8767 (compiled_files|deps|files|exports_file|imports|name|types_sizes) took 969.506346ms 
INFO [runner/filename_unadjuster] Pre-built 0 adjustments in 73.593529ms 
INFO [linters_context/goanalysis] analyzers took 0s with no stages 
INFO [runner/skip_dirs] Skipped 39 issues from dir pkg/app/piped/executor/analysis/mannwhitney by pattern pkg/app/piped/executor/analysis/mannwhitney 
INFO [runner] Issues before processing: 933, after processing: 0 
INFO [runner] Processors filtering stat (in/out): identifier_marker: 412/412, exclude-rules: 412/4, cgo: 933/933, path_prettifier: 933/933, invalid_issue: 933/933, autogenerated_exclude: 815/412, exclude: 412/412, skip_files: 933/854, skip_dirs: 854/815, filename_unadjuster: 933/933, nolint: 4/0 
INFO [runner] processing took 24.417009ms with stages: exclude-rules: 7.225086ms, autogenerated_exclude: 5.916751ms, identifier_marker: 5.327876ms, path_prettifier: 2.180793ms, nolint: 1.832417ms, skip_dirs: 1.165792ms, skip_files: 680.917µs, cgo: 34.417µs, invalid_issue: 33.833µs, filename_unadjuster: 14.792µs, max_same_issues: 1.167µs, sort_results: 834ns, diff: 500ns, uniq_by_line: 375ns, exclude: 332ns, fixer: 292ns, max_from_linter: 250ns, max_per_file_from_linter: 167ns, source_code: 167ns, path_shortener: 84ns, path_prefixer: 84ns, severity-rules: 83ns 
INFO [runner] linters took 310.10779ms with stages: goanalysis_metalinter: 285.621823ms 
INFO File cache stats: 0 entries of total size 0B 
INFO Memory: 15 samples, avg is 47.3MB, max is 128.6MB 
INFO Execution took 1.356511334s             

[Warashi]

changed hard tabs to spaces

[Warashi]

Without this empty line, gofmt converts heading spaces to tabs because they are treated as godoc comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 26.08%. Comparing base (17a5007) to head (25249b6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5435      +/-   ##
==========================================
- Coverage   26.10%   26.08%   -0.02%     
==========================================
  Files         451      451              
  Lines       48837    48837              
==========================================
- Hits        12748    12741       -7     
- Misses      35072    35078       +6     
- Partials     1017     1018       +1     

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[khanhtc1202]

Nice added 👍

[ffjlabo]

🚀