Liveness/Readiness probes implementation

### 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

Currently
### Proposed Solution

To determine whether AdGuard Home process is ready to answer queries, currently I use `nslookup probe 127.0.0.1` to "probe" it -- if I get a `NXDOMAIN` response, it means the instance is ready. In my setup, liveness/readiness probes are necessary to achieve high availability. My setup can be found [here](https://github.com/yuha0/home-infra/tree/master/kubernetes/adguardhome)

Related discussion: https://github.com/AdguardTeam/AdGuardHome/issues/573

### Alternatives Considered

Alternatively, if AdGuard can provide a configuration option to ignore specific request (for example, a query asking for the IP address of "probe"). I can keep using nslookup to perform probes and the query log will stay clean.

### Additional Information