lexgen: support another true unknown field (modTool meta)

lex/type_schema.go

···
 		return "string", nil
 	case "unknown":
 		// NOTE: sometimes a record, for which we want LexiconTypeDecoder, sometimes any object
-		if k == "didDoc" || k == "plcOp" {
+		if k == "didDoc" || k == "plcOp" || k == "meta" {
 			return "interface{}", nil
 		} else {
 			return "*util.LexiconTypeDecoder", nil