XWIKI-11486: Batch API to send large volumes of mail

* Create xwiki-platform-mail-send-batch module structure
* Create iterators (GroupMimeMessageIterator, UsersMimeMessageIterator, SerializedFilesMimeMessageIterator)
We should explain that "Using an Iterator allows to construct the MimeMessage objects one by one and thus allow scaling to an unlimited number of mails to send since we don't need to have all MimeMessage instance in memory at once."