Add default_duo_namespace_id column to the user_preferences table

What does this MR do and why?

References

In this MR is setting up a new user prefences 'Default GitLab Duo Group' to be able to route these request in the cases above.

It validates if the id set is a namespaces where the user has seats assigned.

Query

SELECT "subscription_user_add_on_assignments".* FROM "subscription_user_add_on_assignments"
INNER JOIN "subscription_add_on_purchases" "add_on_purchase" ON "add_on_purchase"."id" = "subscription_user_add_on_assignments"."add_on_purchase_id"
INNER JOIN "subscription_add_ons" ON "subscription_add_ons"."id" = "add_on_purchase"."subscription_add_on_id"
WHERE "subscription_user_add_on_assignments"."user_id" = 12640917
AND "subscription_add_ons"."name" IN (1, 3)
AND "add_on_purchase"."namespace_id" IS NOT NULL

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/41238/commands/126706

MR acceptance checklist

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Related to #552075 (closed)