Users: support authentication

### Current behaviour

Anyone with access to the vehicle's network can access any Cockpit user, which may provide undesirable access to some control interfaces, and potentially information like API keys in custom Actions.

### Expected or desired behaviour

Allow users to be password-protected, and only synchronise the Profiles when authenticated.

Combining this with #1427 could allow for trainees and spectators to have limited access to a vehicle during operation, while an admin / primary operator can create limited interfaces for them with only the functionality they _should_ have access to, and still be able to have full control (with potentially proprietary tools/displays) in a private interface.

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

No files, tests, or entry points are named. Start by tracing the existing Cockpit user and Profile synchronization flow and identify where authentication boundaries belong; done means users can be password-protected and Profiles synchronize only after authentication.

Written by the indexing model from the issue text.