feat(promotions): support copying folders and files for Git

[hiddeco]

Related to #1250 (and more specifically #1250 (comment)).

This adds minimal support for copying files or directories into user-defined paths during a Git promotion. The primary goal of this pull request is to get a feel of how it could work, as the issue appears to be more of a collection of desired outcomes rather than concrete descriptions of functionalities.

spec:
  promotionMechanisms:
    gitRepoUpdates:
    - repoURL: https://github.com/hiddeco/kargo-test.git
      writeBranch: env/dev
      patches:
      # From a Freight commit
      - copy:
          repoURL: https://github.com/example/charts.git
          source: base/values.yaml
          destination: env/dev/values.yaml
      # Within the same repository
      - copy:
          source: base/values.yaml
          destination: env/dev/values.yaml
      render: {}

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[codecov]

Codecov Report

Attention: Patch coverage is 60.49383% with 64 lines in your changes are missing coverage. Please review.

Project coverage is 45.86%. Comparing base (da3c588) to head (783ef6f).
Report is 5 commits behind head on main.

❗ Current head 783ef6f differs from pull request most recent head facf756

Please upload reports for the commit facf756 to get more accurate results.

Files	Patch %	Lines
internal/controller/promotion/git.go	53.15%	46 Missing and 6 partials ⚠️
internal/fs/copy.go	73.91%	6 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2054      +/-   ##
==========================================
+ Coverage   45.84%   45.86%   +0.01%     
==========================================
  Files         236      239       +3     
  Lines       16121    16332     +211     
==========================================
+ Hits         7391     7491     +100     
- Misses       8367     8467     +100     
- Partials      363      374      +11     

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[krancour]

I think we can close this since #2536 has this.