Fixes issues with complex types for phoenix endpoints

• Fixes anyOf/oneOf/allOf defintions for pets, extracts discriminating field from Pet to PetType, but keeps the discrimination definition on Pet. Cats and Dogs will now extend the PetType definition instead of Pet = remove circular reference.
• If x-struct is set in allOf schema, the values will now be castet to the given struct.
• Forward Schema.cast for oneOf/allOf Schemas to Cast.cast()
• Fixes issue with json_render when a error occures on root-level (path: [])
• Implements PhoenixController for pets for the purpose of testing the oneOf/allOf Struct-Casting thing