Map&OrderedMap: use enum type instead of int32 as ordered map keys

[Kybxd]

For the following proto message:

message FruitConf {
  option (tableau.worksheet) = {
    ordered_map: true
  };

  map<int32, Fruit> fruit_map = 1 [(tableau.field) = { name: "Fruit" key: "Key" layout: LAYOUT_INCELL }];
  message Fruit {
    protoconf.FruitType key = 1 [(tableau.field) = { name: "Key" }];
    map<int32, int32> value_list = 3 [(tableau.field) = { name: "Value" layout: LAYOUT_INCELL }];
  }
}

The generated 1-level ordered map getter accepts int32 as func param. It would be better to replace int32 with enum type.

const FruitConf::OrderedMap_int32Map* FruitConf::GetOrderedMap(int32_t key) const {
  const auto* conf = GetOrderedMap();
  if (conf == nullptr) {
    return nullptr;
  }
  auto iter = conf->find(key);
  if (iter == conf->end()) {
    return nullptr;
  }
  return &iter->second.first;
}

// GetOrderedMap1 finds value in the 1-level ordered map. It will return
// NotFound error if the key is not found.
func (x *FruitConf) GetOrderedMap1(key int32) (*OrderedMap_FruitConf_int32Map, error) {
	conf := x.orderedMap
	if val, ok := conf.Get(key); !ok {
		return nil, fmt.Errorf("key(%v) %w", key, ErrNotFound)
	} else {
		return val.First, nil
	}
}

[wenchy]

In fact, it is a breaking change for access APIs.