Automatically upgrade unencrypted DNS systems

### Problem Description

Users do not trust third-party DNS servers and want to randomly use system DNS.

### Proposed Solution

Automatically switch to encrypted DNS for each system DNS server. For example, Wifi A has a DNS of 8.8.8.8, then automatically upgrade to DoH; when switching to wifi B with DNS 1.1.1.1, it will automatically upgrade to DoH; but when switching to wifi C without DoH (like carrier's DNS), continue to use unencrypted DNS.

This feature can be set in developer mode, for testing before official release.

### Additional Information

Chrome browser is currently testing an automatic upgrade feature for encrypted DNS. Details: https://www.chromium.org/developers/dns-over-https

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FAQ
Q: Do you plan to support a canary domain similar to Mozilla's use-application-dns.net?
A: We have no plans to support this approach. We believe that our deployment model is significantly different from Mozilla's, and as a result canary domains won't be needed. In particular, our deployment model is designed to preserve the current user experience, i.e. auto-upgrading to the current DNS provider's DoH server which offers the same features.

Q: How will Chrome's auto-upgrade approach work with Split Horizon?
A: Chrome's auto-upgrade approach does not change the DNS provider, and is designed to preserve the same user experience. Split Horizon setups should continue to work as is. Furthermore, managed deployments should be automatically opted-out, and administrators can use policies to control the feature.
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