[Notifier] add `SentMessageEvent` and `FailedMessageEvent`

[chalasr]

Q	A
Feature PR	symfony/symfony#39601
PR author(s)	@ismail1432
Merged in	5.4

[carsonbot]

Thank you for this issue.
There has not been a lot of activity here for a while. Has this been resolved?

[alamirault]

I think this issue can be close. PR is now merged