Introduce FlashBagAwareSessionInterface

[fabpot]

Q	A
Feature PR	symfony/symfony#46491
PR author(s)	@VincentLanglet
Merged in	6.2

[VincentLanglet]

Hi @fabpot, does this new Interface really need doc ?

[fabpot]

As there are so many questions about this, maybe yes? I don't know.

[javiereguiluz]

Closed in #16936.