Do Protocol (No 7) on the privileges and immunities of the European Union, in particular Article 14 thereof, the principle of a single social security scheme applicable to workers, whether employed or self-employed, active or retired, and the principle of sincere cooperation as set out in Article 4(3) of the Treaty on European Union preclude a Member State from imposing a national social security scheme on, and requiring the payment of social security contributions from, an official who, in addition to his employment within a European institution, also carries out additional teaching activities with the latter’s authorisation, when that official is, by virtue of the Staff Regulations of Officials, already subject to the joint social security scheme of the EU institutions?