Ability to customize payload when sending mail through mailjet+api

[fabpot]

Q	A
Feature PR	symfony/symfony#45152
PR author(s)	@gam6itko
Merged in	6.1

[carsonbot]

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

[alamirault]

Readme was updated in code PR symfony/symfony@9a31686

I think we can close this issue IMO