add operation to balances update configuration to allow specific operation on balance update

conf/modules/billapi/balances.ini

@@ -48,6 +48,8 @@ billapi.balances.update.update_parameters.0.name=value
 billapi.balances.update.update_parameters.0.type=float
 billapi.balances.update.update_parameters.1.name=additional
 billapi.balances.update.update_parameters.1.type=array
+billapi.balances.update.update_parameters.2.name=operation
+billapi.balances.update.update_parameters.2.type=string
 
 billapi.balances.reopen.error_base=60200
 billapi.balances.reopen.query_parameters.0.name=_id