LOG4J2-3332: SmtpAppender - option for sending buffered messages as an email attachment (text, zip, gzip)

[ronosaurus]

DO NOT MERGE: candidate for a 2.19 release after more important changes occur in 2.18

// logEvents.html.zip
smtpBuilder.setAttachEvents(true);
smtpBuilder.setAttachEventsCompression(SmtpManager.AttachEventsCompression.ZIP);

// logEvents.html
smtpBuilder.setAttachEvents(true);

// logEvents.txt.gz
smtpBuilder.setLayout(PatternLayout.createDefaultLayout())
smtpBuilder.setAttachEvents(true);
smtpBuilder.setAttachEventsCompression(SmtpManager.AttachEventsCompression.GZ);