Add casting class for ID Type

This introduces a new Scalar type: GlobalID. This represents a
`GlobalID` value, which can be passed to `GitlabSchema.object_from_id`
to load an object.

app/graphql/types/global_id_type.rb

+# frozen_string_literal: true
+
+module Types
+  class GlobalIDType < GraphQL::Schema::Scalar
+    description "A wrapper for GraphQL IDs"
+
+    # @param value [GID]
+    # @return [String]
+    def self.coerce_result(value, _ctx)
+      value.to_s
+    end
+
+    # @param value [String]
+    # @return [GID]
+    def self.coerce_input(value, _ctx)
+      GlobalID.parse(value.to_s)
+    rescue ArgumentError, TypeError
+      # Invalid input
+      nil
+    end
+  end
+end