Custom IPv6/DHCPv6 prefixlen

### 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

### Description

#### What problem are you trying to solve?

Trying to get DHCPv6 up and running, but it fails me due to the wrong prefixlen.

#### Proposed solution

Option to set the prefixlen in the DHCP settings

#### Alternatives considered

Not using AGH as DHCP, but what's the purpose in having it then? The function built-in I mean ...

#### Additional information

I'm facing a bit of an issue here ...

My FRITZ!Box as the Gateway has it's own IPv6 address with a prefixlen of `/64` which I cannot change at all. AGH seems to hand out IPv6 with a prefixlen of `/128`. Meaning, all the devices in my network that receive their IPv6 from AGH and do not have them set statically, can't communicate, because they are in a completely different (sub)network.

This conundrum makes IPv6 a bit unusable for me at the moment.

AGH DHCPv6 Settings

DHCPv6 Auto Configuration from AGH

Result when trying to use IPv6 via AGH DHCPv6:

So it would be nice if we could have an option to set the right prefixlen for IPv6, since this is not supported by every Gateway/Router (FRITZ!Box in my case).

Thank you!