client: update DNSConfig type (#8916)

This PR adds DNSFilterURL to the DNSConfig type to be used by
control changes to add DNS filtering logic

Fixes #cleanup

Signed-off-by: Richard Castro <richard@tailscale.com>

client/tailscale/apitype/controltype.go

@@ -10,6 +10,7 @@ type DNSConfig struct {
 	Domains           []string                 `json:"domains"`
 	Nameservers       []string                 `json:"nameservers"`
 	Proxied           bool                     `json:"proxied"`
+	DNSFilterURL      string                   `json:"DNSFilterURL"`
 }
 
 type DNSResolver struct {