Support plugin auth and mysql_clear_password for AWS IAM authentication

[kyleaa]

AWS added support for connecting using short-lived IAM credentials instead of long-lived username/passwords. This is implemented using the MySQL Pluggable Authentication Protocol and requires the token be sent in cleartext.

http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.IAMDBAuth.Connecting.AWSCLI.html

https://dev.mysql.com/doc/refman/5.5/en/cleartext-pluggable-authentication.html

I have a working implementation of this, but I am very unfamiliar with the internals of mariaex and the MySQL dev docs were a bit lacking, so it was created through trial and error. I can put it up for PR if that is desired.

https://github.com/xerions/mariaex/compare/master...kyleaa:aurora