lnrpc: expose forwarding channel for htlcs in ListChannels

lnrpc/rpc.proto

@@ -1009,6 +1009,18 @@ message HTLC {
 
     // Index identifying the htlc on the channel.
     uint64 htlc_index = 5;
+
+    // If this HTLC is involved in a forwarding operation, this field indicates
+    // the forwarding channel. For an outgoing htlc, it is the incoming channel.
+    // For an incoming htlc, it is the outgoing channel. When the htlc
+    // originates from this node or this node is the final destination,
+    // forwarding_channel will be zero. The forwarding channel will also be zero
+    // for htlcs that need to be forwarded but don't have a forwarding decision
+    // persisted yet.
+    uint64 forwarding_channel = 6;
+
+    // Index identifying the htlc on the forwarding channel.
+    uint64 forwarding_htlc_index = 7;
 }
 
 enum CommitmentType {

lnrpc/rpc.swagger.json

@@ -3818,6 +3818,16 @@
           "type": "string",
           "format": "uint64",
           "description": "Index identifying the htlc on the channel."
+        },
+        "forwarding_channel": {
+          "type": "string",
+          "format": "uint64",
+          "description": "If this HTLC is involved in a forwarding operation, this field indicates\nthe forwarding channel. For an outgoing htlc, it is the incoming channel.\nFor an incoming htlc, it is the outgoing channel. When the htlc\noriginates from this node or this node is the final destination,\nforwarding_channel will be zero. The forwarding channel will also be zero\nfor htlcs that need to be forwarded but don't have a forwarding decision\npersisted yet."
+        },
+        "forwarding_htlc_index": {
+          "type": "string",
+          "format": "uint64",
+          "description": "Index identifying the htlc on the forwarding channel."
         }
       }
     },