lnrpc/rpc.proto: add mpp fields to Hop message

This commit add mpp_total_amt_msat and mpp_payment_addr to the Hop
message. Doing so enables users submitting mpp payments via rpc to
set these parameters for the destination. In addition, it will allow us
to display these fields in rpc responses.

lnrpc/rpc.proto

@@ -1836,6 +1836,32 @@ message Hop {
     TLV format.
     */
     bool tlv_payload = 9 [json_name = "tlv_payload"];
+
+    /**
+    An optional TLV record tha singals the use of an MPP payment. If present,
+    the receiver will enforce that that the same mpp_record is included in the
+    final hop payload of all non-zero payments in the HTLC set. If empty, a
+    regular single-shot payment is or was attempted.
+    */
+    MPPRecord mpp_record = 10 [json_name = "mpp_record"];
+}
+
+message MPPRecord {
+    /**
+    A unique, random identifier used to authenticate the sender as the intended
+    payer of a multi-path payment. The payment_addr must be the same for all
+    subpayments, and match the payment_addr provided in the receiver's invoice.
+    The same payment_addr must be used on all subpayments.
+    */
+    bytes payment_addr = 11 [json_name = "payment_addr"];
+
+    /**
+    The total amount in milli-satoshis being sent as part of a larger multi-path
+    payment. The caller is responsible for ensuring subpayments to the same node
+    and payment_hash sum exactly to total_amt_msat. The same
+    total_amt_msat must be used on all subpayments.
+    */
+    int64 total_amt_msat = 10 [json_name = "total_amt_msat"];
 }
 
 /**

lnrpc/rpc.swagger.json

@@ -2530,6 +2530,10 @@
           "type": "boolean",
           "format": "boolean",
           "description": "* \nIf set to true, then this hop will be encoded using the new variable length\nTLV format."
+        },
+        "mpp_record": {
+          "$ref": "#/definitions/lnrpcMPPRecord",
+          "description": "*\nAn optional TLV record tha singals the use of an MPP payment. If present,\nthe receiver will enforce that that the same mpp_record is included in the\nfinal hop payload of all non-zero payments in the HTLC set. If empty, a\nregular single-shot payment is or was attempted."
         }
       }
     },
@@ -2873,6 +2877,21 @@
         }
       }
     },
+    "lnrpcMPPRecord": {
+      "type": "object",
+      "properties": {
+        "payment_addr": {
+          "type": "string",
+          "format": "byte",
+          "description": "*\nA unique, random identifier used to authenticate the sender as the intended\npayer of a multi-path payment. The payment_addr must be the same for all\nsubpayments, and match the payment_addr provided in the receiver's invoice.\nThe same payment_addr must be used on all subpayments."
+        },
+        "total_amt_msat": {
+          "type": "string",
+          "format": "int64",
+          "description": "*\nThe total amount in milli-satoshis being sent as part of a larger multi-path\npayment. The caller is responsible for ensuring subpayments to the same node\nand payment_hash sum exactly to total_amt_msat. The same\ntotal_amt_msat must be used on all subpayments."
+        }
+      }
+    },
     "lnrpcMacaroonPermission": {
       "type": "object",
       "properties": {