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

* Create xwiki-platform-mail-send-batch module structure
* Create iterators (GroupMimeMessageIterator, UsersMimeMessageIterator, SerializedFilesMimeMessageIterator)
I don't think this should be here. Either we set this globally for all modules and it should be in the top level pom (which is a good idea IMO) or it should not be used.