Allow customers to require licensed seats for Duo Chat

Summary

After Chat is GA (April 18, 2024) but before the licensed cut-off date (TBD), some Gitlab.com customers only want users with a Duo Pro licensed seat to be able to use Chat.

To allow this, we will introduce a feature flag that is enabled for root groups. Members of that group will only be able to use Chat if they have a licensed seat.

Implementation details

Using the new duo_chat_requires_licensed_seat feature flag:

• if the user belongs to any group with the feature flag enabled and has a Duo Pro add-on seat, they can use Duo Chat.
• if the user belongs to any group with the feature flag enabled and does not have a Duo Pro add-on seat, they cannot use Duo Chat.

Feature flag rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/457091