Auto-enable Duo Code Review on first Duo add-on purchase

What does this MR do and why?

Stamp enable_duo_code_review_by_default as pending in GitlabCom::ProvisionService when a namespace acquires its first Duo add-on, so EnableCodeReviewFlowWorker enables the setting and the code_review/v1 flow. Follows the enable_duo_core_for_new_subscription hook in the same class.

Database

SELECT 1 AS one
FROM subscription_add_on_purchases
WHERE namespace_id = $1
  AND subscription_add_on_uid IN (5, 1, 3, 7)
  AND (started_at IS NULL OR started_at <= $2)
  AND ($2 < expires_on)
LIMIT 1;

References

Screenshots or screen recordings

How to set up and validate locally

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