Add shareable directive to interfaces

Author: Ross Cooperman

lib/apollo-federation/interface.rb

@@ -22,6 +22,10 @@ def inaccessible
         add_directive(name: 'inaccessible')
       end
 
+      def shareable
+        add_directive(name: 'shareable')
+      end
+
       def tag(name:)
         add_directive(name: 'tag', arguments: [name: 'name', values: name])
       end