AddOnPurchase process trial attribute

GitlabSubscriptions::AddOnPurchase introduced a new field `trial`. This
new field needs to be handled in API methods and update service:
- Update the add-on purchase for the namespace API accepts the
  `trial` as optional param
- Create an add-on purchase for the namespace API accepts the
  `trial` as optional param
- EE::API::Entities::GitlabSubscriptions::AddOnPurchase expose `trial`
- GitlabSubscriptions::AddOnPurchases::UpdateService updates `trial`

Changelog: added
EE: true