types/dnstype: introduce new package for Resolver

So the type can be used in net/dns without introducing a tailcfg
dependency.

For #2596

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

types/dnstype/dnstype.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package dnstype defines types for working with DNS.
+package dnstype
+
+//go:generate go run tailscale.com/cmd/cloner --type=Resolver --clonefunc=true --output=dnstype_clone.go
+
+import "inet.af/netaddr"
+
+// Resolver is the configuration for one DNS resolver.
+type Resolver struct {
+	// Addr is the address of the DNS resolver, one of:
+	//  - A plain IP address for a "classic" UDP+TCP DNS resolver
+	//  - [TODO] "tls://resolver.com" for DNS over TCP+TLS
+	//  - [TODO] "https://resolver.com/query-tmpl" for DNS over HTTPS
+	Addr string `json:",omitempty"`
+
+	// BootstrapResolution is an optional suggested resolution for the
+	// DoT/DoH resolver, if the resolver URL does not reference an IP
+	// address directly.
+	// BootstrapResolution may be empty, in which case clients should
+	// look up the DoT/DoH server using their local "classic" DNS
+	// resolver.
+	BootstrapResolution []netaddr.IP `json:",omitempty"`
+}