some sql classes refactorings like column and table structs to classes

include/matador/sql/query_context.hpp

@@ -34,6 +34,7 @@ struct query_context {
   std::string sql;
   sql_command command{};
   std::string command_name{};
+  std::string schema_name{};
   sql::table table{""};
   std::vector<object::attribute_definition> prototype{};
   std::vector<std::string> result_vars{};