Add control of user 2F preference

User's credential panel on HomeUI should allow for setting whether the user opts in for 2nd factor. UI - simple checkbox. Additionally there should be an information this is only a preference and may be overwritten by system rules.

The checkbox should not be visible when user has at least 2 valid credentials.

Admin should have a HomeUI setting to control whether this checkbox should be enabled or not.

State of the checkbox should be stored in system attribute (to be created) and used when endpoint has 2nd factor policy userOptIn.