[VarExporter] Add trait to help implement lazy loading ghost objects

[nicolas-grekas]

Q	A
Feature PR	symfony/symfony#46751
PR author(s)	@nicolas-grekas
Merged in	6.2

[wouterj]

Hi Nicolas. I'm afraid this and #17249 are way beyond the knowledge that we documentors have about Symfony code.
Are you willing to write up a nice section (maybe new article in the var exporter component?) about the proxy features provided by Symfony 6.2? 🙏