Add output schema in plan node

[lgbo-ustc]

There is only names in RelRoot. But we need a exact definition of the output schema in some cases.

What about adding a new field of struct to define the output schema in RelRoot ?

message RelRoot {
  // A relation
  Rel input = 1;
  // Field names in depth-first order
  repeated string names = 2;
  Type.Struct output_schema = 3;
}