Add trial column to subscription_add_on_purchases table

What does this MR do and why?

In issue https://gitlab.com/gitlab-org/gitlab/-/issues/448506+, we discussed and agreed that: adding a trial field to GitlabSubscriptions::AddOnPurchase table is useful to clearly tells whether a pack is from trial order, or from the paid subscription.

This MR adds trial column to subscription_add_on_purchases table

This column tells whether the AddOn pack is from trial order or from paid subscription. Default to false.

Changelog: added EE: true

Database

Output from db:migrate

% bundle exec rake db:migrate:main
main: == [advisory_lock_connection] object_id: 120060, pg_backend_pid: 54111
main: == 20240320062459 AddTrialToSubscriptionAddOnPurchases: migrating =============
main: -- add_column(:subscription_add_on_purchases, :trial, :boolean, {:default=>false, :null=>false})
main:    -> 0.0017s
main: == 20240320062459 AddTrialToSubscriptionAddOnPurchases: migrated (0.0048s) ====

main: == [advisory_lock_connection] object_id: 120060, pg_backend_pid: 54111

Output from db:rollback

% bundle exec rake db:migrate:down:main VERSION=20240320062459
main: == [advisory_lock_connection] object_id: 119780, pg_backend_pid: 54437
main: == 20240320062459 AddTrialToSubscriptionAddOnPurchases: reverting =============
main: -- remove_column(:subscription_add_on_purchases, :trial, :boolean, {:default=>false, :null=>false})
main:    -> 0.0012s
main: == 20240320062459 AddTrialToSubscriptionAddOnPurchases: reverted (0.0055s) ====

main: == [advisory_lock_connection] object_id: 119780, pg_backend_pid: 54437

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