lnrpc+rpcserver: Add blinded payment paths to PayReq

This commit adds a blinded_paths field to the PayReq proto message. A
new helper called `CreateRPCBlindedPayments` is then added to convert
the zpay32 type to the existing `lnrpc.BlindedPaymentPath` type and add
this to the `PayReq` in the `DecodePayReq` rpc method.

lnrpc/lightning.swagger.json

@@ -6299,6 +6299,12 @@
           "additionalProperties": {
             "$ref": "#/definitions/lnrpcFeature"
           }
+        },
+        "blinded_paths": {
+          "type": "array",
+          "items": {
+            "$ref": "#/definitions/lnrpcBlindedPaymentPath"
+          }
         }
       }
     },