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
We need to decide if we stop the mail sending if one error happens when creating one message. One idea is to continue and have the status attached to each mail so that it can be viewed in the Admin UI. Now it's likely that if it fails to create one message it'll fail to create all the others too so it might be correct to report only a single error. I'm curious to see how this surfaces in the scripting API...