Add multi-plugin authentication

MariaDB already implements a mechanism that allows multiple plugins to be defined for a user that can all be used to authenticate the user (logical OR). Extending the syntax to allow multiple required authentication plugins would make it compatible with MySQL (logical AND).

Given plugins foo, bar, baz the following is possible:

CREATE USER markus IDENTIFIED VIA foo OR bar OR baz;

The request is to implement:

CREATE USER markus IDENTIFIED VIA foo AND bar AND baz;

MySQL implements this in a more fixed manner where there can be up to three mechanisms. Extending the MariaDB system to support both AND and OR would make it more versatile:

CREATE USER markus IDENTIFIED VIA mysql_native_password AS PASSWORD('my-password') AND pam OR unix_socket;

The example above would identify the user via mysql_native_password as well as pam or if it's a local connection, only via the unix_socket plugin.