Best-effort mode for Parental Control and Safe Browsing

### Prerequisites

- [X] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) and found no answer

- [X] I have searched other issues and found no duplicates

- [X] I want to report a bug and not [ask a question or ask for help](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a)

- [X] I have set up AdGuard Home correctly and [configured clients to use it](https://github.com/AdguardTeam/AdGuardHome/wiki/Clients). (Use the [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) for help with installing and configuring clients.)

### Platform (OS and CPU architecture)

EdgeRouter EdgeOS mips64_softfloat

### Installation

systemd

### Setup

On a router, DHCP is handled by the router

### AdGuard Home version

v0.107.36

### Action

When `family-block.dns.adguard.com` or `standard-block.dns.adguard.com` are down, AGH is very slow or completely unusable, DNS requests time out.

Can you please add a "best effort" option for the security and parental control web services where AGH stops using them for a minute if it detects they are down, then tries them again once in a while until they are back up again? Some sort of a healthcheck.

I realize some users want strict enforcement of these options, but others are fine with a "best effort" use, if they are down no big deal, continue serving DNS.

### Expected result

AGH should at least continue to serve DNS.

### Actual result

AGH is unusable.

### Additional information and/or screenshots

_No response_