Add option to configure smtp debug printer

[michal-krajewski]

Hello!
The default behaviour of Jakarta Mail is to push smtp debug logs to std out. It's not a viable solution for my use case; I need to redirect these logs to the Slf4j logger.
To achieve that, I'm accessing Session object directly and setting a custom printer:
mailer.getSession().setDebugOut(new Slf4jPrinter(logger));
(the printer is my custom implementation)

Enhancement:
Allow configuring the debug printer directly in the configuration
.withDebugPrinter(printer)