Add metafields top level key

[shauns]

WHY are these changes introduced?

Allows developer to specify an API version while testing

WHAT is this pull request doing?

Adds the top-level key required. The contract validates the content.

How to test your changes?

Run deploy with an app with some test configuration.

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• Existing analytics will cater for this addition
• PR includes analytics changes to measure impact

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[github-actions]

Thanks for your contribution!

Depending on what you are working on, you may want to request a review from a Shopify team:

• Themes: @shopify/advanced-edits
• UI extensions: @shopify/ui-extensions-cli
  • Checkout UI extensions: @shopify/checkout-ui-extensions-api-stewardship
• Hydrogen: @shopify/hydrogen
• Other: @shopify/app-inner-loop

[github-actions]

We detected some changes at either packages/*/src or packages/cli-kit/assets/cli-ruby/** and there are no updates in the .changeset.
If the changes are user-facing, run "pnpm changeset add" to track your changes and include them in the next release CHANGELOG.

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	72.82% (+0.11% 🔼)	8249/11328
🟡	Branches	69.48% (+0.05% 🔼)	4022/5789
🟡	Functions	71.5% (+0.03% 🔼)	2152/3010
🟡	Lines	73.17% (+0.11% 🔼)	7804/10666

Test suite run success

1860 tests passing in 845 suites.

Report generated by 🧪jest coverage report action from 873ea2b