lnrpc: add htlc attempt id

lnrpc/routerrpc/router.swagger.json

@@ -524,6 +524,11 @@
     "lnrpcHTLCAttempt": {
       "type": "object",
       "properties": {
+        "attempt_id": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The unique ID that is used for this attempt."
+        },
         "status": {
           "$ref": "#/definitions/HTLCAttemptHTLCStatus",
           "description": "The status of the HTLC."