fix(serializer): Pass the child context when denormalizing nested non-resource objects

[philipsorst]

Q	A
Branch?	2.6
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	n/a
License	MIT
Doc PR	n/a

Currently, the createChildContext() method isn't used when denormalizing embedded objects that are not resources. Hence, for example, the AbstractNormalizer::ATTRIBUTES won't work for these objects.

This PR fixes that by invoking createChildContext() also for nested non-resource objects.

This is closely related to #4258 and #4521 which fixed the same issue when normalizing.

[soyuka]

as https://github.com/api-platform/core/pull/4521 broke quite a few implementations I'd like to move this over to the main branch with a rebase please.

[stale[bot]]

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