[Mailer] Add a way to inject Stamps when sending an email via Messenger

[fabpot]

Q	A
Feature PR	symfony/symfony#47191
PR author(s)	@fabpot
Merged in	6.2

[wouterj]

Hi @fabpot! I'm not sure I can completely follow the changes, whether this removes the MessageEvent or adds an additional event.

I guess we need to add/change https://symfony.com/doc/current/mailer.html#mailer-events Are you able to help us out? :)

[fabpot]

@wouterj See #17390