Make private_networks editable in the UI

### 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

If i attempt any private reverse lookups, they fail unless i do the lookup explicitly over IPv4. This is because I have IPv6 deployed. I'm also unable to get a reverse lookup for an Ipv6 address over either protocol.

It sounds like i need to add my IPv6 prefix to private_networks in AdGuardHome.yaml.

The problem is, i'm using the Home Assistant add-on, which doesn't expose AdGuardHome.yaml.

### Proposed solution

Add private_networks to the UI

### Alternatives considered and additional information

Submitted a request for the Home Assistant addon to expose this, but they're adamant that this should be an upstream feature and they don't want to expose AdGuardHome.yaml. Basically we'd have to fork the addon, if you don't implement it here.