Frontend: Add Secrets Manager Open Beta settings toggle for self-managed

Why are we doing this work

We need to add an admin settings toggle for self-managed instances that allows administrators to enroll in the Secrets Manager Open Beta. The toggle state will be queryable and modified via GraphQL mutation and when enabled will allow for Secrets Manager feature availability instance wide.

The setting itself will be hidden behind the open beta feature flags, to be determined in https://gitlab.com/gitlab-org/gitlab/-/work_items/597896. It's likely we'll use different feature flags for instances and TLG.

Relevant links

Refer to https://gitlab.com/gitlab-org/gitlab/-/work_items/597481+ for the designs.

Refer to https://gitlab.com/gitlab-org/gitlab/-/work_items/597896+ for the feature flag.

Implementation plan

Verification steps

1. Upload a Premium license (or above).
2. Set up openbao and enable the secrets manager on your gdk.
3. Enable the Open Beta feature flag (to be provided in https://gitlab.com/gitlab-org/gitlab/-/work_items/597896).
4. Go to your project settings (Settings > General) and expand Visibility, project features, permissions. Verify that the secrets manager toggle does not exist yet in the settings.
5. Go to the Admin settings and enable the secrets manager open beta toggle.
6. Go back to the permissions settings of your project and verify that the secrets manager toggle is now available.