lnrpc: add msats to return value of DecodePayReq

In this commit, we add `msats` to the return value of `DecodePayReq` to
ensure we always show full value information as we're moving to do
generally for all RPC calls that deal with off-chain amounts.

lnrpc/rpc.proto

@@ -2616,6 +2616,7 @@ message PayReq {
     int64 cltv_expiry = 9 [json_name = "cltv_expiry"];
     repeated RouteHint route_hints = 10 [json_name = "route_hints"];
     bytes payment_addr = 11 [json_name = "payment_addr"];
+    int64 num_msat = 12 [json_name = "num_msat"];
 }
 
 message FeeReportRequest {}

lnrpc/rpc.swagger.json

@@ -3252,6 +3252,10 @@
         "payment_addr": {
           "type": "string",
           "format": "byte"
+        },
+        "num_msat": {
+          "type": "string",
+          "format": "int64"
         }
       }
     },