Refactor labels management from HAML to Vue - Add `subscribed` field to GrahpQL query

We need to be able to know if a user has subscribed to a label or not. We should add the subscribed field to https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/graphql/types/label_type.rb

    field :subscribed, GraphQL::Types::Boolean, null: false,
          description: 'If current user is subscribed to the label.'

    def subscribed
      object.subscribed?(current_user)
    end