Port check_proxy_users system variable and related server feature from MySQL 5.7

MySQL 5.7 added the check_proxy_users system variable and related server feature. When this variable is enabled, the server performs proxy user mappings. This allows proxy users to be used with an authentication plugin, even if the authentication plugin itself does not map proxy users.

The mysql_native_password_proxy_users system variable can be used to enable/disable this behavior for the mysql_native_password authentication plugin.

If we port this feature, then maybe we would also want similar system variables to enable/disable this behavior for other authentication plugins, like ed25519 or gssapi.

https://dev.mysql.com/doc/refman/5.7/en/proxy-users.html

https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_check_proxy_users

https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_mysql_native_password_proxy_users