DHCPv6 options for nameservers

### 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

override any nameservers given by the router's RA which points to itself or ISP's and therefore bypassing AdGuard for name resolutions over IPv6 (which turns out to be heavily used by e.g. Windows clients, for all kind of records, A and AAAA alike, even though a IPv4 server is available)
or simply provide one if not given by anything else.

Unlike DHCPv4 there doesn't seem to be any way to configure options (e.g. nameservers) for DHCPv6 to distribute to clients, without necessarily providing IPv6 addresses as such.

Likewise, there's no visible option to send RA with such options.

### Proposed solution

Feature that both sends RA and DHCPv6 responses to clients and effectively override the router's RA,
at least on some OS, as DHCPv6 options seems to be preferred.

If this already exists, could not figure it out or find it in the docs.

Pihole has such a feature called _Enable IPv6 support (SLAAC + RA)_

### Alternatives considered and additional information

_No response_