lnd: add the option to set the incoming channel on blinded path

This commit makes sure that when an incoming channel list is
supplied for the blinded route, that it changes the other blinding
restrictions accordingly. Moreover it introduces a sanity check when
the number of paths for the blinded route is set to 0.

rpcserver.go

@@ -6055,6 +6055,7 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 		NumHops:                  globalBlindCfg.NumHops,
 		MaxNumPaths:              globalBlindCfg.MaxNumPaths,
 		NodeOmissionSet:          fn.NewSet[route.Vertex](),
+		IncomingChainedChannels:  make([]uint64, 0),
 	}
 
 	if blindCfg != nil && !blind {
@@ -6074,6 +6075,10 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 			blindingRestrictions.MaxNumPaths =
 				uint8(*blindCfg.MaxNumPaths)
 		}
+		if blindingRestrictions.MaxNumPaths == 0 {
+			return nil, fmt.Errorf("blinded max num paths cannot " +
+				"be 0")
+		}
 
 		for _, nodeIDBytes := range blindCfg.NodeOmissionList {
 			vertex, err := route.NewVertexFromBytes(nodeIDBytes)
@@ -6083,6 +6088,38 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 
 			blindingRestrictions.NodeOmissionSet.Add(vertex)
 		}
+
+		blindingRestrictions.IncomingChainedChannels = append(
+			blindingRestrictions.IncomingChainedChannels,
+			blindCfg.IncomingChannelList...,
+		)
+
+		numChainedChannels :=
+			uint8(len(blindingRestrictions.IncomingChainedChannels))
+
+		// When selecting the blinded incoming channel list parameter
+		// the maximum number of hops is implictitly set.
+		if numChainedChannels > blindingRestrictions.NumHops {
+			rpcsLog.Warnf("Changing the num_blinded_hops "+
+				"from (%d) to (%d)",
+				blindingRestrictions.NumHops,
+				numChainedChannels)
+
+			blindingRestrictions.NumHops =
+				numChainedChannels
+		}
+
+		// The MinDistanceFromIntroNode must be greater than or equal to
+		// the number of hops specified on the chained channels.
+		minNumHops := blindingRestrictions.MinDistanceFromIntroNode
+		if minNumHops < numChainedChannels {
+			return nil, fmt.Errorf("minimum number of blinded "+
+				"path hops (%d) must be greater than or equal "+
+				"to the number of hops specified on the "+
+				"chained channels (%d)", minNumHops,
+				numChainedChannels)
+		}
+
 	}
 
 	if blindingRestrictions.MinDistanceFromIntroNode >