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

* Create xwiki-platform-mail-send-batch module structure
* Create iterators (GroupMimeMessageIterator, UsersMimeMessageIterator, SerializedFilesMimeMessageIterator)
[minor] should be above "-default" since we sort alphabetically, you should also add the message that it's sorted alphabetically (see other pom.xml files).