Add the serializer service
merged branch loalf/add_serializer_service (PR #6815)

This PR was merged into the master branch.

Discussion
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[2.3] [FrameworkBundle] [Serializer] Loads the Serializer component as a service in the Framework Bundle

This PR is the same as
https://github.com/symfony/symfony/pull/5347

but since I am struggling to squash all the commits I better create a new one. Sorry for the inconveniences, :)

Commits
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b4e4844 Add the serializer service