plugin/view: Advanced routing interface and new 'view' plugin (#5538)

* introduce new interface "dnsserver.Viewer", that allows a plugin implementing it to decide if a query should be routed into its server block.
* add new plugin "view", that uses the new interface to enable a user to define expression based conditions that must be met for a query to be routed to its server block.

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/pkg/replacer/replacer_test.go

@@ -340,13 +340,12 @@ func TestMetadataReplacement(t *testing.T) {
 		Next: next,
 	}
 
-	m.ServeDNS(context.TODO(), &test.ResponseWriter{}, new(dns.Msg))
-	ctx := next.ctx // important because the m.ServeDNS has only now populated the context
-
 	w := dnstest.NewRecorder(&test.ResponseWriter{})
 	r := new(dns.Msg)
 	r.SetQuestion("example.org.", dns.TypeHINFO)
 
+	ctx := m.Collect(context.TODO(), request.Request{W: w, Req: r})
+
 	repl := New()
 	state := request.Request{W: w, Req: r}