add mysql2 option jsonStrings option to return MySQL Json field as string

mysql2 connector add a new option jsonStrings, in order to return JSON field as string in place of JSON.
For compatibility, this option must be added.

connector already has an option `autoJsonMap`, but this will only limit mariadb with 10.5.2 to return either JSON or string.
jsonStrings will ensure that MySQL JSON field as well as autoJsonMap will return string or JSON