Add extension to store oneOf/anyOf/allOf objects as full properties

Created by: zippolyte

oneOf, anyOf and allOf in a composed schema only store the names of the different objects, so it's not possible to do complex things in the templates depending on things like type of object (things like isArray, isEnum, isPrimitive, ...).

This PR converts the objects to properties and stores them in the vendor extensions x-(oneOf|anyOf|allOf)-as-properties to be used in templates.

cc @wing328 @jimschubert, would appreciate some feedback on this, thanks 🙂

I plan on using them in custom templates for now, so I have no template change in this PR, but I feel this could be useful to have here.

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