some sql classes refactorings like column and table structs to classes

include/matador/sql/record.hpp

@@ -8,7 +8,7 @@
 
 namespace matador::sql {
 
-struct column;
+class column;
 
 class record final {
 private:
@@ -41,7 +41,7 @@ public:
 
   [[nodiscard]] const std::vector<field_ref>& columns() const;
 
-  [[nodiscard]] const field& at(const column &col) const;
+  [[nodiscard]] const field& at(const sql::column &col) const;
   [[nodiscard]] const field& at(size_t index) const;
   template<class Type>
   std::optional<Type> at(const column &col) const {