support get ipv6 prefix from interface

### 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

For users utilizing PPPOE, the IPv6 prefix may be dynamic. Consequently, they need to adjust the prefix to match that of the interface or employ other software such as `dnsmasq` to address this issue.
I hope AdGuardHome can consider adding a feature to retrieve the IPv6 prefix from an interface. It shouldn't be necessary to use DHCP-PD to obtain an IPv6 prefix from a specific interface. This functionality can be implemented by other software, but it's not mandatory for AdGuardHome. Additionally, it would be beneficial if AdGuardHome could invalidate the advertised prefix when the prefix changes on the interface.

### Proposed solution

In dnsmasq, you can configure it to tackle this issue as follows:
```
dhcp-range=tag:enp1s0,::1,constructor:enp1s0, ra-names, 10m
enable-ra
```
It will contact dnsmasq to obtain the prefix from the 'enp1s0' interface, which is added by dhcpcd. The dhcpcd configuration looks like this:
```
duid
noipv6rs
waitip 6
ipv6only
interface ppp0
ipv6rs
iaid 1
ia_pd 1 enp1s0
```

### Alternatives considered and additional information

AdGuard's DHCP server function is extremely useful for those who don't use a router system like OpenWRT. Configuring IPv6 can be a bit challenging for newcomers, and perhaps some people don't prefer using multiple pieces of software to complete this task.