More information about connectors support for MariaDB features

(Idea from a discussion with @grooverdan) - Rename the `Extensions` column to `Features`; - Add features that are not MariaDB-specific; - In the features description, state `MariaDB-only` where appropriate. Examples of not-only-Maria features that should be added: - PAM authentication; - load-data-infile; - exposure of some of the OK packet status/messages.