exp: Add GitHub Actions Executor

[yottahmd]

This experimental feature will enable access to thousands of pre-built GitHub Actions available in the marketplace.

To-do:

• Implement handling for action outputs
• Add documentation clearly indicating that this is an experimental feature
• Add integration tests
• Verify compatibility in the DinD environment

Example:

workingDir: /tmp/workspace
steps:
  - uses: actions/checkout@v4
    with:
      repository: dagu-org/dagu
      ref: main
      token: ${GITHUB_TOKEN}

[cloudflare-workers-and-pages]

Deploying dagu-doc with    Cloudflare Pages

Latest commit:	810299b
Status:	✅  Deploy successful!
Preview URL:	https://d6a63e83.dagu-doc.pages.dev
Branch Preview URL:	https://poc-github-actions.dagu-doc.pages.dev

View logs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.65%. Comparing base (17e7193) to head (810299b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1309   +/-   ##
=======================================
  Coverage   63.64%   63.65%           
=======================================
  Files         142      142           
  Lines       21046    21048    +2     
=======================================
+ Hits        13395    13398    +3     
- Misses       6592     6595    +3     
+ Partials     1059     1055    -4     

Files with missing lines	Coverage Δ
internal/digraph/builder/builder.go	66.01% <100.00%> (+0.05%)	⬆️
internal/digraph/step.go	91.52% <ø> (ø)

... and 5 files with indirect coverage changes

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