Handle messages with fields omitted

[davidjgoss]

🤔 What's changed?

Use parseEnvelope from the messages library that deserialises all envelopes into classes that have default values for numbers (0), arrays ([]) and strings (""), rather than a plain JSON.parse.

⚡️ What's your motivation?

We can see from telemetry that some client-side errors are happening when reports are rendered for things like:

• A Timestamp with undefned nanos
• A Scenario with undefined examples

Whilst I think it's a bad thing in the long term (and have been trying to get rid of it, prematurely in retrospect), we need to have it for now so we tolerate missing fields, and then ideally establish the gaps with schema adherence and close them over time.

🏷️ What kind of change is this?

• 🐛 Bug fix (non-breaking change which fixes a defect)

📋 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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[mpkorstanje]

LGTM