routerrpc: add amp bool to SendPayment

For now this is how you indicate you want to perform an AMP payment to
the destination.

lnrpc/routerrpc/router.proto

@@ -279,6 +279,11 @@ message SendPaymentRequest {
     value is in milli-satoshis.
     */
     uint64 max_shard_size_msat = 21;
+
+    /*
+    If set, an AMP-payment will be attempted.
+    */
+    bool amp = 22;
 }
 
 message TrackPaymentRequest {