[Serializer] Add `SerializedPath` annotation to flatten nested attribut…

[fabpot]

Q	A
Feature PR	symfony/symfony#43534
PR author(s)	@boenner
Merged in	6.2

[wouterj]

Hi @boenner. Thank you for contributing this feature! Do you have time and motivation to also write some docs about it? :)

I think the best place would be a new section right after ["Using Serialization Groups Attributes""](https://symfony.com/doc/6.2/serializer.html#using-serialization-groups-attributes).