Failover support

Hello,

as a follow-up to feature request https://github.com/AdguardTeam/AdGuardHome/issues/790, I would like to suggest adding failover support at a later date. I propose to solve this task in two ways:
1. A bundle of two or more servers with equal priority. In this case, each of the servers is aware of the existence of the other;
2. A bundle of two or more servers, but the first will have an marked as Primary (main) priority, and the second and subsequent marked as Secondary (child) etc.

Yes, formally, at hte moment this problem can be solved simply using type a few A records for a domain and several server entities distributed in different locations physically. But I'm offering a single endpoint of control them.

It may also solve all problems with anycast support in the future?... Please, consider this probable function🤔

Thank you.
Cheers😜