Add plugin implementation

This can be used as an alternative to pipeline modification.

lib/middlewares/object_scope_authorization.ex

@@ -62,12 +62,17 @@ defmodule Rajska.ObjectScopeAuthorization do
   alias Absinthe.{Blueprint, Phase, Type}
   alias Rajska.Introspection
   use Absinthe.Phase
+  @behaviour Absinthe.Plugin
 
   @spec run(Blueprint.t() | Phase.Error.t(), Keyword.t()) :: {:ok, map}
   def run(%Blueprint{execution: execution} = bp, _options \\ []) do
     {:ok, %{bp | execution: process(execution)}}
   end
 
+  def pipeline(pipeline, _execution), do: pipeline
+  def before_resolution(execution), do: execution
+  def after_resolution(execution), do: process(execution)
+
   defp process(%{validation_errors: [], result: result} = execution), do: %{execution | result: result(result, execution.context)}
   defp process(execution), do: execution
 
@@ -80,6 +85,9 @@ defmodule Rajska.ObjectScopeAuthorization do
     result
   end
 
+  # No fields because of non_null violation further down the tree
+  defp result(%{fields: nil} = result, _context), do: result
+
   # Root
   defp result(%{fields: fields, emitter: %{schema_node: %{identifier: identifier}}} = result, context)
   when identifier in [:query, :mutation, :subscription] do