Add furthest step state

...sets/javascripts/subscriptions/buy_addons_shared/utils.js

...scripts/subscriptions/graphql/queries/state.query.graphql

ee/app/assets/javascripts/subscriptions/new/graphql.js

.../javascripts/vue_shared/purchase_flow/components/step.vue

...flow/graphql/queries/furthest_accessed_step.query.graphql

...javascripts/vue_shared/purchase_flow/graphql/resolvers.js

...scripts/vue_shared/purchase_flow/graphql/typedefs.graphql

...re_flags/gitlab_com_derisk/key_contacts_management_v2.yml

ee/lib/ee/gitlab/gon_helper.rb

ee/spec/frontend/subscriptions/mock_data.js

...criptions/new/components/checkout/billing_address_spec.js

.../purchase_flow/components/checkout/payment_method_spec.js

...ue_shared/purchase_flow/components/checkout/zuora_spec.js

...frontend/vue_shared/purchase_flow/components/step_spec.js

...ontend/vue_shared/purchase_flow/graphql/resolvers_spec.js

ee/spec/frontend/vue_shared/purchase_flow/spec_helper.js