A better AC/Serializers integration API

[joaomdmoura]

tl;dr

Implement a first-class serializer interface in Rails much like there is for renderers.
Proposed interfaces are: serializer_for and ActionController::Serializers.register

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This is something @bf4 is working on. Great work btw.
It will probably be handle most on Rails, but this issue will help us to keep track of it on our side.

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related to issues:

• Add configurable serializers in each renderer rails/rails#21496 (rails)

related to PRs:

• ...