java: Convert message to and from maps

[mpkorstanje]

🤔 What's changed?

⚡️ What's your motivation?

Currently only Jackson can convert messages to json. Other serialization libraries such as Gson require that the messages are mutable. We can work around this by reading to/from a map with primitives.

This makes it possible to extend cucumber/messages-ndjson#19 with other serialization libraries.

🏷️ What kind of change is this?

• 📖 Documentation (improvements without changing code)
• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)
• 🐛 Bug fix (non-breaking change which fixes a defect)
• ⚡ New feature (non-breaking change which adds new behaviour)
• 💥 Breaking change (incompatible changes to the API)

♻️ Anything particular you want feedback on?

📋 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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