[RFC] add HydrationFactory as extension point for tracking hydrations

[kbond]

(Continuation of #9545)

This is a proposed extension point to help solve doctrine/DoctrineBundle#109. I'm thinking a HydratorInterface that AbstractHydrator implements should be added but wanted to get some feedback/input before going further.

The doctrine bundle could use this to add hydration times to the timeline:

The doctrine bundle could add something similar to https://github.com/debesha/DoctrineProfileExtraBundle#screenshots to the profiler panel.

You can see an example of how this could be used in DoctrineBundle here: https://github.com/kbond/symfony-reproducer/tree/hydration-profiler-poc (specifically kbond/symfony-reproducer@9a4e91b).

I don't think this causes any performance problems for hydration (when using the DefaultHydratorFactory).

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