Enable Usage of Multiple Configuration 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

Currently AdGuardHome supports only a single designated configuration file, however across multiple deployments it is often useful to have a specific set of core configuration along with a series of deployment specific configurations with the ability to easily update the core configuration without touching the deployment specific config.

### Proposed solution

The ability to define multiple configuration files of varying precedence would solve this problem as then any given configuration file can be updated completely independent of any others.

### Alternatives considered and additional information

_No response_