add required field to enhance ecommerce schema

[jeffreylean]

[snowplowcla]

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[jeffreylean]

Thanks for your pull request. Is this your first contribution to a Snowplow open source project? Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://docs.snowplowanalytics.com/docs/contributing/contributor-license-agreement/ to learn more and sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

I signed it!

[snowplowcla]

Confirmed! @jeffreylean has signed the Contributor License Agreement. Thanks so much.

[paulboocock]

Hey @jeffreylean Thanks for the PR.

Could you point me to any Google Documentation which confirms that these properties are required 100% of the time on these objects? That'd help me with considering approving this change. If there is any other motivation behind this change, then that would also be good context.