lnrpc: add htlc index to ListChannels pending htlcs

lnrpc/rpc.proto

@@ -1006,6 +1006,9 @@ message HTLC {
     int64 amount = 2;
     bytes hash_lock = 3;
     uint32 expiration_height = 4;
+
+    // Index identifying the htlc on the channel.
+    uint64 htlc_index = 5;
 }
 
 enum CommitmentType {

lnrpc/rpc.swagger.json

@@ -3813,6 +3813,11 @@
         "expiration_height": {
           "type": "integer",
           "format": "int64"
+        },
+        "htlc_index": {
+          "type": "string",
+          "format": "uint64",
+          "description": "Index identifying the htlc on the channel."
         }
       }
     },