[Mailer] Extract transport factory and allow create custom transports

[fabpot]

Q	A
Feature PR	symfony/symfony#31946
PR author(s)	@Koc
Merged in	4.4

[carsonbot]

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

[carsonbot]

Friendly ping? Should this still be open? I will close if I don't hear anything.