Admin Control for Duo CLI
### User problem to solve
As a top-level group owner, I want Duo CLI off for my namespace until it's been reviewed and approved, so that it isn't available to my developers before we've cleared it through security and procurement.
### Proposal
Add a binary setting in `admin/gitlab_duo/configuration`
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#### **Default behavior**
* The setting defaults to ~~`off`~~ `on` for all instances, including those where Duo CLI was previously used in beta. We'll treat the beta to GA boundary as a clean break rather than a per-instance migration.
#### State logic
Duo CLI is available only when all of the following are true.
| Duo availability | Agent Platform | CLI toggle | Duo CLI result |
|------------------|----------------|------------|----------------|
| Always off | any | any | unavailable |
| Off by default | any | any | unavailable |
| On | Off | any | unavailable |
| On | On | any | Unavailable |
| On | On | Off | Unavailable |
| On | On | On | Available |
#### Acceptance Criteria
* setting is visible on the admin configuration page, above Agents section
* setting defaulted to ~~off~~ on
* when the setting is off, Duo CLI requests return an error directing the user to contact their admin
* when any parent setting is off (see table above), the CLI toggle is disabled in the UI
* beta users transitioning with us to GA land in the off state and see the admin-contact error until an admin enables the setting
* Duo CLI docs callout that admin enablement is required, with a link to admin configuration page
#### What we need to figure out
* Error message copy shown to users when Duo CLI has not been enabled by admin
issue