lnd+lnrpc: update fwdinghistory message

In this commit we update the returned message for fwdinghistory
to include the htlcindex for all forwarded htlcs.

lnrpc/lightning.proto

@@ -4716,6 +4716,14 @@ message ForwardingEvent {
     // The peer alias of the outgoing channel.
     string peer_alias_out = 13;
 
+    // The ID of the incoming HTLC in the payment circuit. This field is
+    // optional and is unset for forwarding events happened before v0.20.
+    optional uint64 incoming_htlc_id = 14;
+
+    // The ID of the outgoing HTLC in the payment circuit. This field is
+    // optional and may be unset for legacy forwarding events.
+    optional uint64 outgoing_htlc_id = 15;
+
     // TODO(roasbeef): add settlement latency?
     //  * use FPE on the chan id?
     //  * also list failures?

lnrpc/lightning.swagger.json

@@ -5075,6 +5075,16 @@
         "peer_alias_out": {
           "type": "string",
           "description": "The peer alias of the outgoing channel."
+        },
+        "incoming_htlc_id": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The ID of the incoming HTLC in the payment circuit. This field is\noptional and is unset for forwarding events happened before v0.20."
+        },
+        "outgoing_htlc_id": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The ID of the outgoing HTLC in the payment circuit. This field is\noptional and may be unset for legacy forwarding events."
         }
       }
     },