feat: add support to generate UE DataTable: ue-csv and ue-json

[wenchy]

References

• Data Driven Gameplay Elements: https://docs.unrealengine.com/5.1/en-US/data-driven-gameplay-elements-in-unreal-engine/
• DataTable: https://docs.unrealengine.com/5.1/en-US/BlueprintAPI/EditorScripting/DataTable/
• UE4 / Unreal Engine 4 / Data Tables (CSV & JSON): https://www.youtube.com/watch?v=ZXq_ntrfojI

Design

Extend Metasheet in proto/tableau/protobuf/metabook.proto:

message Metasheet {
  ...
  Mode mode = 17 [(tableau.field) = { name: "Mode" optional: true }];
}

enum Mode {
  MODE_DEFAULT = 0; // Default mode.
  // UE DataTable references:
  //  - https://docs.unrealengine.com/5.1/en-US/data-driven-gameplay-elements-in-unreal-engine/
  //  - https://docs.unrealengine.com/5.1/en-US/BlueprintAPI/EditorScripting/DataTable/
  MODE_UE_CSV = 1; // CSV format of UE DataTable.
  MODE_UE_JSON= 2; // JSON format of UE DataTable.
  MODE_ENUM_TYPE = 3; // Enum type definition in sheet.
  MODE_UNION_TYPE = 4; // Union type definition in sheet.
}

For example, metasheet @TABLEAU in workbook:

Sheet	Mode
Skill	MODE_UE_CSV
Buff	MODE_UE_JSON