Function toResource needs to see the target content-type

[bruderj15]

Currently we have

class ToResource res a where
  toResource :: Proxy res -> a -> res a

instance Resource res => ToResource res User where
  toResource _ usr = addRel ("address", mkAddrLink $ addressId usr) $ wrap usr
    where
      mkAddrLink = toRelationLink (Proxy @AddressGetOne)

However in mkAddrLink we want to point to the address-resource, not the old endpoint from the plain api.
Therefore we need access to the content type so we can ref to the hateoas-endpoint.

class ToResource ct a where
  toResource :: Proxy ct -> Proxy a -> MkResource ct a

instance Resource (MkResource ct) => ToResource ct User where
  toResource _ usr = addRel ("address", mkAddrLink $ addressId usr) $ wrap usr
    where
      mkAddrLink = toRelationLink (Proxy @(Resourcify AddressGetOne ct))