Support getmail imap idle option

It would be great if the getmail imap idle function would be supported so that IMAP mailboxes do not need to be polled every 5 minutes but mails will arrive immediately.

As this would need to run a system service for every getmail rcconfig that needs imap idle, it would be a bigger change in how ISPconfig handles the getmail configuration.

See https://pyropus.ca./software/getmail/configuration.html#running-commandline-options and https://work-work.work/blog/2018/12/15/getmail-systemd-imap-idle.html