[BE] Modify member_roles factory to take :billable trait and create billable member_role

Currently creating billable member_role in spec is not a clear process, it relies on the idea of using capability which we know is billable and non-billable. It would be ideal to just pass :billable to member_role create and let the factory handle which capability is billable.

eg:

  let_it_be(:billable_role) do
    create(
      :member_role,
      :guest,
      namespace: nil,
      read_vulnerability: true
    )
  end

  let_it_be(:non_billable_role) do
    create(
      :member_role,
      :guest,
      namespace: nil,
      read_code: true
    )
  end

Once we have modified the factory, it would be nice to change usage of factory to make use of the new trait