routing: add outgoing channel restriction

lnrpc/rpc.proto

@@ -774,6 +774,12 @@ message SendRequest {
     send the payment.
     */
     FeeLimit fee_limit = 8;
+
+    /**
+    The channel id of the channel that must be taken to the first hop. If zero,
+    any channel may be used.
+    */
+    uint64 outgoing_chan_id = 9;
 }
 message SendResponse {
     string payment_error = 1 [json_name = "payment_error"];

lnrpc/rpc.swagger.json

@@ -2903,6 +2903,11 @@
         "fee_limit": {
           "$ref": "#/definitions/lnrpcFeeLimit",
           "description": "*\nThe maximum number of satoshis that will be paid as a fee of the payment.\nThis value can be represented either as a percentage of the amount being\nsent, or as a fixed amount of the maximum fee the user is willing the pay to\nsend the payment."
+        },
+        "outgoing_chan_id": {
+          "type": "string",
+          "format": "uint64",
+          "description": "*\nThe channel id of the channel that must be taken to the first hop. If zero,\nany channel may be used."
         }
       }
     },