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

* Create xwiki-platform-mail-send-storage module structure
* Create iterators (GroupMimeMessageIterator, UsersMimeMessageIterator, SerializedFilesMimeMessageIterator)
* Add factories for different iterators
* Rename MailResultListener and create Component
* Change returned type of MailSender#send and MailSender#sendAsynchronously
I'm not sure it's the best to pass File objects here. I'd save all the files related to a batch in a directory named after the batch id and I'd pass the File handle to that directory. I believe this scales better than passing tons of File objects...