When using native DNS, notify the user that they have to manually enable AdGuard in iOS DNS settings when toggling on AdGuard protection

### Prerequisites

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- [X] I am running the latest version
- [X] I checked the documentation and found no answer
- [X] I checked to make sure that this issue has not already been filed

### Problem Description

When using native DNS and toggling AdGuard off, then back on, AdGuard will reenable AdGuard as the DNS provider in iOS DNS settings as this requires user intervention.
The issue with this is that there is no way for the user to know that this is required at the moment without checking the DNS state.

### Proposed Solution

Notify the user with the same message as when toggling on native DNS protection from the DNS protection page in the AdGuard app.
My proposed solution is as below.


### Alternatives Considered

### Additional Information

If a user is using a custom DNS server with the native DNS implementation, AdGuard will not return as an option in iOS DNS settings until a preconfigured DNS provider is selected in the AdGuard app, before switching back to the custom DNS server. Refer to #2203 as to why.