[Serializer] Add context builders

[chalasr]

Q	A
Feature PR	symfony/symfony#43973
PR author(s)	@mtarld
Merged in	6.1

[nicolas-grekas]

Friendly ping @mtarld, this really needs doc.

[mtarld]

Yes, I'll try to do it ASAP

[javiereguiluz]

Closing as fixed by #16591.