Add a "Random" option to the DNS settings

### Discussed in https://github.com/AdguardTeam/AdGuardHome/discussions/4748

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<sup>Originally posted by **dontcrash** July 16, 2022</sup>
![image](https://user-images.githubusercontent.com/32660358/179344161-e54a4476-57a3-43c2-9413-dbaecd498102.png)
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