Add AttachmentExternalisingStream

[davidjgoss]

🤔 What's changed?

Adds a new stream class for externalising attachments per some given options.

This can precede CucumberHtmlStream in a pipeline that produces an HTML report, externalising attachments as needed.

⚡️ What's your motivation?

cucumber/cucumber-js#2781

🏷️ What kind of change is this?

• ⚡ New feature (non-breaking change which adds new 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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