validate: support assigning validate to MODE_STRUCT_TYPE_MULTI and MODE_UNION_TYPE_MULTI sheets

[wenchy]

Based on #378

Should add MessageProp (like FieldProp) to tableau.proto:

tableau/proto/tableau/protobuf/tableau.proto

Lines 268 to 276 in fe37408

	message FieldProp {
	// Usage: {range:"1,10" refer:"Item.ID"}
	// Different interpretations of range:
	// - number: value range
	// - string: count of utf-8 code point
	//
	// Format: "1,10", "1,~", "~,10".
	string range = 1;

message MessageProp {
   // Protovalidate message-level constraints. The value is a text-format
  // representation of buf.validate.MessageConstraints.
  //
  // See https://github.com/bufbuild/protovalidate.
  string validate = 1;
}

Extract and assign to option (buf.validate.message).

For example:

Tree	Tree note	{validate:"cel:{id:\"tree.id_required\" message:\"id must not be default\" expression:\"this.id!=0\"}"}
Number	Name	Type
1	ID	uint32
2	Num	int32
Pet	Pet note
Number	Name	Type
1	Kind	int32
2	Tip	[]string