Ability to settings via Environment Variables

### 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

Love the project! I run it in standalone Docker containers.

I would like to be able to inject users into the container, at start up time, via environment variables. It'd also be great, if other, specific settings could be injected this same way.

This would allow me to persist configuration files to source control, w/o having to worry about secrets being exposed.

### Proposed solution

The most flexible, I think, would be to create a syntax, where `AdGuardHome.yaml` could be configured to look in an environment variable for a value.

Potential syntax:

```yaml
users:
- name: user1
password:
- name: ${ADGUARD_HOME_USER2_USERNAME}
password: ${ADGUARD_HOME_USER2_PASSWORD
```

At launch, by populating the ENV VARS `ADGUARD_HOME_USER2_USERNAME` and `ADGUARD_HOME_USER2_PASSWORD`, these values would be used. If not value is found, have the program crash at startup.

### Alternatives considered and additional information

_No response_