Add DNS64 settings to web 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

### Description

#### Description

In #5117 we've added support for DNS64 in accordance with [RFC 6147](https://datatracker.ietf.org/doc/html/rfc6147). That feature added the `dns64_prefixes` setting into `dns` object of the configuration.

We should expose the setting in the HTTP API, add the UI, as well as adjust the current description of the "Private reverse DNS servers" field.