Client allowlist/blocklist for admin page access

### 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

All devices can access the admin page as of now.

### Proposed solution

It'd be great if a feature can be introduced to only allow/disallow specific clients from accessing the admin page.

Either a new list can be introduced with admin privileges or the existing client list can be utilized.

It is also important to ensure a safeguard so that one does not lock himself out while changing this setting.

### Alternatives considered and additional information

_No response_