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.swagger.json

@@ -1493,6 +1493,11 @@
           "type": "string",
           "format": "uint64",
           "description": "The largest payment split that should be attempted when making a payment if\nsplitting is necessary. Setting this value will effectively cause lnd to\nsplit more aggressively, vs only when it thinks it needs to. Note that this\nvalue is in milli-satoshis."
+        },
+        "amp": {
+          "type": "boolean",
+          "format": "boolean",
+          "description": "If set, an AMP-payment will be attempted."
         }
       }
     },