[Feature request] Allow more DNS-over-HTTPS providers

It looks like that currently only certain DoH providers are allowed: see the following chunk of code in https://cs.android.com/android/platform/superproject/+/master:packages/modules/DnsResolver/PrivateDnsConfiguration.h;drc=3aea8db351212126ee1420598090057c258f8335;l=259

    // TODO: Move below DoH relevant stuff into Rust implementation.
    std::map<unsigned, DohIdentity> mDohTracker GUARDED_BY(mPrivateDnsLock);
    std::array<DohProviderEntry, 5> mAvailableDoHProviders = {{
            {"Google",
             {"2001:4860:4860::8888", "2001:4860:4860::8844", "8.8.8.8", "8.8.4.4"},
             "dns.google",
             "https://dns.google/dns-query",
             false},
            {"Google DNS64",
             {"2001:4860:4860::64", "2001:4860:4860::6464"},
             "dns64.dns.google",
             "https://dns64.dns.google/dns-query",
             false},
            {"Cloudflare",
             {"2606:4700::6810:f8f9", "2606:4700::6810:f9f9", "104.16.248.249", "104.16.249.249"},
             "cloudflare-dns.com",
             "https://cloudflare-dns.com/dns-query",
             false},

            // The DoH providers for testing only.
            // Using ResolverTestProvider requires that the DnsResolver is configured by someone
            // who has root permission, which should be run by tests only.
            {"ResolverTestProvider",
             {"127.0.0.3", "::1"},
             "example.com",
             "https://example.com/dns-query",
             true},
            {"AndroidTesting",
             {"192.0.2.100"},
             "dns.androidtesting.org",
             "https://dns.androidtesting.org/dns-query",
             false},
    }};

It would be nice to also allow other DoH providers, like NextDNS.

The logic behind choosing between DoH and DoTLS does not actually look trivial (see calls to setDot and setDoh from PrivateDnsConfiguration::set in https://cs.android.com/android/platform/superproject/+/master:packages/modules/DnsResolver/PrivateDnsConfiguration.cpp;drc=bf52841c57f05b9caff97c80d2d86d59ecf609e4;l=115), and maybe it's even better to just have two separate options in private DNS configuration: one for those who want to use DoT, and one for DoH.

Actually, one field (as right now) should be enough, and choice between DoH/DoT can be made just because resolver addresses look different (in case of NextDNS, https://dns.nextdns.io/username vs username.dns.nextdns.io). I'm not sure why it's not implemented this way right now since it seems pretty straightforward. This way, probably there would be no need for "white list" of allowed DoH resolvers, and usage of any resolver would be possible (if I'm not missing smth).

Thanks for your great work on GrapheneOS!

GrapheneOS / os-issue-tracker

[Feature request] Allow more DNS-over-HTTPS providers #3827