feat: add config for checksum during project build

[Marcus Müller (M-arcus)]

What changed?

Added two new options to the project config (.shopware-project.yml) to control extension checksum generation during shopware-cli project ci:

• build.disable_checksums when true, the "Generating extension checksums" step is skipped entirely, so no checksum.json files are written into the extensions.
• build.keep_existing_checksums when true, extensions that already ship a checksum.json keep it untouched; extensions without one still get a generated file.

Both default to false, existing behavior is unchanged.

build:
  disable_checksums: true
  keep_existing_checksums: true

Why?

Some extensions already deliver a checksum.json and I want to keep those intact. keep_existing_checksums covers that case, disable_checksums is for pipelines that don't want the files generated at all.

How was this tested?

new tests added (internal/shop/config_test.go)

Related issue or discussion

No.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.92%. Comparing base (2991cdd) to head (e911f0f).

Files with missing lines	Patch %	Lines
cmd/project/ci.go	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1293      +/-   ##
==========================================
- Coverage   53.93%   53.92%   -0.02%     
==========================================
  Files         303      303              
  Lines       23400    23405       +5     
==========================================
  Hits        12620    12620              
- Misses      10752    10757       +5     
  Partials       28       28              

Flag	Coverage Δ
go-test	53.92% <0.00%> (-0.02%)	⬇️

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