lnd+rpcserver: allow external subservers to specify macaroon validator

Give the external subservers the possibility to also use their own
validator to check any macaroons attached to calls to their registered
gRPC URIs.
This allows them to have their own root key ID database and permission
entities.

lnd.go

@@ -145,6 +145,12 @@ type RPCSubserverConfig struct {
 	// per URI, they are all required. See rpcserver.go for a list of valid
 	// action and entity values.
 	Permissions map[string][]bakery.Op
+
+	// MacaroonValidator is a custom macaroon validator that should be used
+	// instead of the default lnd validator. If specified, the custom
+	// validator is used for all URIs specified in the above Permissions
+	// map.
+	MacaroonValidator macaroons.MacaroonValidator
 }
 
 // ListenerWithSignal is a net.Listener that has an additional Ready channel that