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
Note that I think that we should reimplement this method without going through an async process but this can be done later. Also we should use a POJO for the MailListener to be able to call getErrors()-type methods