[Maria-developers] Please review: MDEV-7199 Introduce the concept of data type handlers

Hi Sergei, while working on addressing your review suggestions for "MDEV-4912 Add a plugin to field types (column types)", I decided to create a separate tasks for a piece of additional changes: https://mariadb.atlassian.net/browse/MDEV-7199 MDEV-7199 Introduce the concept of data type handlers Please review. Thanks.