Update protobuf schema for Elixir

[davidjgoss]

🤔 What's changed?

Manually update the protobuf definition in the elixir module so that the published package includes the new Pickle location field from #308.

⚡️ What's your motivation?

Elixir is still being generated from the protobuf definition per #63, so changing the JSON schema and regenerating code in #308 didn't work for Elixir, and this throws up a problem in cucumber/gherkin#433 when trying to make the corresponding Gherkin change. I'll tackle the code generation separately, but this at least gets Elixir to parity right now.

🏷️ What kind of change is this?

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

♻️ Anything particular you want feedback on?

I've slotted in the new field before other fields in the order which matches the JSON schema change but is a no-no in protobuf - but I think this is okay because we're only using the protobuf definition to generate code and not to to (de)serialisation of data in the protobuf way.

📋 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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[davidjgoss]

After discussing offline, we're going to go a different way here - the messages and gherkin impls for elixir will be frozen until #63 is done.