Add ops feature flag to disable Arkose challenge

What does this MR do and why?

Closes gitlab-com/gl-infra/production-engineering#25655 (closed)

Due to an incident with Arkose, users were unable to complete signup via oauth. This MR adds an ops feature flag so that Arkose can be quickly disabled if there are issues with the service.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Start GDK with SaaS simulation

   $ export GITLAB_SIMULATE_SAAS=1
   $ gdk start

2. Configure the required application settings. Credentials can be found in 1Password in under ArkoseLabs API keys. The development keys should be used.

   > ::Gitlab::CurrentSettings.update(email_confirmation_setting: 'hard')
   > ::Gitlab::CurrentSettings.update(require_admin_approval_after_user_signup: false)
   > ::Gitlab::CurrentSettings.update(arkose_labs_public_api_key: 'PUBLIC_KEY', arkose_labs_private_api_key: 'PRIVATE_KEY', arkose_labs_namespace: 'client')

3. Navigate to the signup page. You should see that a challenge is required in order to register a new user.

4. Disable the feature flag

   > Feature.disable(:arkose_labs)

5. Refresh the registration page. You should no longer see the Arkose challenge.