Enable custom samples and add a parallel one

[davidjgoss]

🤔 What's changed?

• Add some sensible plumbing for easily adding custom (non-CCK) samples
• Add a custom sample for a realistic-ish parallel execution

⚡️ What's your motivation?

Provide a more useful sample for working on #524, and make this kidn of thing easier in future.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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