net/dns/{publicdns,resolver}: add NextDNS DoH support

NextDNS is unique in that users create accounts and then get
user-specific DNS IPs & DoH URLs.

For DoH, the customer ID is in the URL path.

For IPv6, the IP address includes the customer ID in the lower bits.

For IPv4, there's a fragile "IP linking" mechanism to associate your
public IPv4 with an assigned NextDNS IPv4 and that tuple maps to your
customer ID.

We don't use the IP linking mechanism.

Instead, NextDNS is DoH-only. Which means using NextDNS necessarily
shunts all DNS traffic through 100.100.100.100 (programming the OS to
use 100.100.100.100 as the global resolver) because operating systems
can't usually do DoH themselves.

Once it's in Tailscale's DoH client, we then connect out to the known
NextDNS IPv4/IPv6 anycast addresses.

If the control plane sends the client a NextDNS IPv6 address, we then
map it to the corresponding NextDNS DoH with the same client ID, and
we dial that DoH server using the combination of v4/v6 anycast IPs.

Updates #2452

Change-Id: I3439d798d21d5fc9df5a2701839910f5bef85463
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager_test.go

@@ -562,6 +562,30 @@ func TestManager(t *testing.T) {
 					"bradfitz.ts.com.", "2.3.4.5"),
 			},
 		},
+		{
+			name: "corp-v6",
+			in: Config{
+				DefaultResolvers: mustRes("1::1"),
+			},
+			os: OSConfig{
+				Nameservers: mustIPs("1::1"),
+			},
+		},
+		{
+			// This one's structurally the same as the previous one (corp-v6), but
+			// instead of 1::1 as the IPv6 address, it uses a NextDNS IPv6 address which
+			// is specially recognized.
+			name: "corp-v6-nextdns",
+			in: Config{
+				DefaultResolvers: mustRes("2a07:a8c0::c3:a884"),
+			},
+			os: OSConfig{
+				Nameservers: mustIPs("100.100.100.100"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(".", "2a07:a8c0::c3:a884"),
+			},
+		},
 	}
 
 	trIP := cmp.Transformer("ipStr", func(ip netip.Addr) string { return ip.String() })