Iteration 1.0, Automatic Duo pro assignment via a namespace setting
## Background
In addition to bulk assignment functionality, one nice-to-have feature for admins would be auto-assignment to new users added to the namespace when available Duo seats exist.
WHEN -
* A new user is created or added to the namespace, AND
* They meet the standard Duo user eligibility criteria, AND
* Duo seats are available to be assigned
THEN - auto-assign a Duo Pro seat to the new user(s), up until no available seats remain.
Example from GitHub - https://docs.github.com/en/enterprise-cloud@latest/copilot/managing-copilot-for-business/managing-access-for-copilot-for-business-in-your-organization#enabling-access-to-github-copilot-for-all-current-and-future-users-in-your-organization
# Proposal
**Use Case:** As a group owner, I want to ensure any **future-added users** are automatically assigned a Duo Pro seat upon user creation so I don't have to continuously manage seat assignments.
This should be setup as a namespace setting - if enabled, then auto-assign Duo to new users.
# Status
Work on this initiative is ~90% complete but has been moved to ~"workflow::blocked". A holistic review of all AI settings is in progress with the AI UX team, and we would like to continue that effort prior to releasing this functionality.
For now, we will keep the feature behind a feature flag until we can validate the customer need and determine what additional features (if any) would be needed to make it valuable for customers.
See [this thread](https://gitlab.com/groups/gitlab-org/-/epics/13637#note_1981965239) for more context.
epic