Allow reading passwords from files

### Prerequisites

- [X] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions) and found no answer

- [X] I have searched other issues and found no duplicates

- [X] I want to request a feature or enhancement and not ask a question

### The problem

Integration with declarative configuration and secret management systems is complicated because passwords are in the same file as the rest of the configuration.

### Proposed solution

It would be nice to be able to specify passwords via files:

```yaml
users:
- name: some_user
password_file: /run/secrets/user2-addguard-bcrypt-password.txt
```

Something similar should also be allowed for the `http_proxy` password.

### Alternatives considered and additional information

Note that, while in some way similar to what proposed in #6647, this requires no special syntax.

Any chance a PR implementing this would be accepted?