Add attr_reader :options for TypeMembership

[mpospelov]

In order to filter-out interfaces-type membership during ApolloFederation dumping the access to TypeMembership#options is opened.

The use case is the following:

1. We have schema like

class Foobar < BaseType
  implements FooInterface
end

2. In order to drop it in 0 downtime fashion we need to mark this implements

class Foobar < BaseType
  implements FooInterface, apollo_federation: false
end

3. So in order to access this apollo_federation key value we need to open access to TypeMembership#options instance variable

[rmosolgo]

Thanks for the improvement here!