State immunity — Employment — Immunity in disputes arising out of an employment contract with an embassy — United Nations Convention on Jurisdictional Immunities of States and Their Property, 2004 — Article 11(1) and (2)(a) of Jurisdictional Immunities Convention constituting customary international law — Whether employee was employed to perform sovereign activities — Whether determinative — Whether States enjoying immunity regarding disputes with employees performing activities closely functionally related to diplomatic and consular functions of the State — The law of Austria