DHCP: Specify the list of DNS servers

### 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, when using AdGuard DHCP, the DNS servers are automatically set to the DHCP server's IP. Twice.

On a simple (and usually weak) home architecture, relying only on the DNS of an home automation mini PC is dangerous for the whole network, potentially also preventing the cloud alarm to notify the user.

### Proposed solution

For better reliability of the network (surviving a short crash or reboot of the machine hosting AdGuard), **additional DNS should be possible to list through DHCP**.

AdGuard protection would then be inactive for this moment, this is important to note, but a warning and a checkbox would make the administrator responsible for this choice.

### Alternatives considered and additional information

Would be very useful when, as it is the case for me, AdGuards primary use on a server is not the protection but the DHCP+DNS features.