Possibility to specify a list of "allowed clients"

### 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

As of now, the list of "Allowed clients" has to be managed manually, by adding IP or IP ranges one by one.

For instance, if one wants to limit access to the ADGH just to IPs from some Country or some Internet provider (as in my case), the list has to be modified periodically "by hand" or one has to fiddle with the AdguarHome.yml file, which is quite uncomfortable.

### Proposed solution

It would be useful to add the possiblity to dynamically load an "Allowed Clients" list from an external source, be it a URL or a local file, with a timer to automatically update the list, with the same mechanism of DNS blocklist.

Thus, on the server side, some script may periodically update an `allowed.txt` list (using `whois.radb.net`, for instance). In turn, ADGH instance would periodically check the `allowed.txt` list for changes.

Thanks a lot in advance.

### Alternatives considered and additional information

_No response_