Allow Serializer#attributes and #relationships to be a function, with access to action instance

[davewasmer]

This will allow customization of what is sent over the wire based on the action context, i.e. limited public fields for user listing, but admins see everything.

[davewasmer]

This will also allow i.e. global serializer attributes:

class FooSerializer extends ApplicationSerializer {
  attributes() {
    super.attributes().concat([ 'foo', 'bar' ])
  }
}