Add instance setting to enable or disable Duo Nano features

What does this MR do and why?

This change adds a new column to enable or disable Duo Nano features on an instance level. The new setting will also be used to control the display of a banner about the Duo Nano availability. The following values need to be supported:

• true: Duo Nano features are enabled.
• false: Duo Nano features are disabled.
• null: No action was taken.

References

https://gitlab.com/gitlab-org/gitlab/-/issues/533435+

Migrations

Up migration

main: == [advisory_lock_connection] object_id: 134060, pg_backend_pid: 18508
main: == 20250407080000 AddDuoNanoFeaturesEnabledToApplicationSettings: migrating ===
main: -- add_column(:application_settings, :duo_nano_features_enabled, :boolean, {:null=>true})
main:    -> 0.0022s
main: == 20250407080000 AddDuoNanoFeaturesEnabledToApplicationSettings: migrated (0.0066s) 

main: == [advisory_lock_connection] object_id: 134060, pg_backend_pid: 18508
ci: == [advisory_lock_connection] object_id: 134360, pg_backend_pid: 18510
ci: == 20250407080000 AddDuoNanoFeaturesEnabledToApplicationSettings: migrating ===
ci: -- add_column(:application_settings, :duo_nano_features_enabled, :boolean, {:null=>true})
ci:    -> 0.0018s
ci: == 20250407080000 AddDuoNanoFeaturesEnabledToApplicationSettings: migrated (0.0109s) 

ci: == [advisory_lock_connection] object_id: 134360, pg_backend_pid: 18510

Down migration(s)

main: == [advisory_lock_connection] object_id: 133740, pg_backend_pid: 17671
main: == 20250407080000 AddDuoNanoFeaturesEnabledToApplicationSettings: reverting ===
main: -- remove_column(:application_settings, :duo_nano_features_enabled, :boolean, {:null=>true})
main:    -> 0.0027s
main: == 20250407080000 AddDuoNanoFeaturesEnabledToApplicationSettings: reverted (0.0087s) 

ci: == [advisory_lock_connection] object_id: 133740, pg_backend_pid: 18099
ci: == 20250407080000 AddDuoNanoFeaturesEnabledToApplicationSettings: reverting ===
ci: -- remove_column(:application_settings, :duo_nano_features_enabled, :boolean, {:null=>true})
ci:    -> 0.0030s
ci: == 20250407080000 AddDuoNanoFeaturesEnabledToApplicationSettings: reverted (0.0144s) 

How to set up and validate locally

1. A green test suite should be sufficient for this change.
2. Manual testing can be done by updating and checking the new column:

Gitlab::CurrentSettings.duo_nano_features_enabled # => nil
Gitlab::CurrentSettings.duo_nano_features_enabled? # => false

Gitlab::CurrentSettings.update(duo_nano_features_enabled: false)
Gitlab::CurrentSettings.duo_nano_features_enabled? # => false

Gitlab::CurrentSettings.update(duo_nano_features_enabled: true)
Gitlab::CurrentSettings.duo_nano_features_enabled? # => true

Gitlab::CurrentSettings.update(duo_nano_features_enabled: nil) # reset to nil

MR acceptance checklist

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